Thread: Use streaming read API in ANALYZE

Hi,

On Mon, 19 Feb 2024 at 18:13, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
>
> I worked on using the currently proposed streaming read API [1] in ANALYZE. The patch is attached. 0001 is the not
yetmerged streaming read API code changes that can be applied to the master, 0002 is the actual code.
 
>
> The blocks to analyze are obtained by using the streaming read API now.
>
> - Since streaming read API is already doing prefetch, I removed the #ifdef USE_PREFETCH code from
acquire_sample_rows().
>
> - Changed 'while (BlockSampler_HasMore(&bs))' to 'while (nblocks)' because the prefetch mechanism in the streaming
readAPI will advance 'bs' before returning buffers.
 
>
> - Removed BlockNumber and BufferAccessStrategy from the declaration of scan_analyze_next_block(), passing pgsr
(PgStreamingRead)instead of them.
 
>
> I counted syscalls of analyzing ~5GB table. It can be seen that the patched version did ~1300 less read calls.
>
> Patched:
>
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  39.67    0.012128           0     29809           pwrite64
>  36.96    0.011299           0     28594           pread64
>  23.24    0.007104           0     27611           fadvise64
>
> Master (21a71648d3):
>
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  38.94    0.016457           0     29816           pwrite64
>  36.79    0.015549           0     29850           pread64
>  23.91    0.010106           0     29848           fadvise64
>
>
> Any kind of feedback would be appreciated.
>
> [1]:
https://www.postgresql.org/message-id/CA%2BhUKGJkOiOCa%2Bmag4BF%2BzHo7qo%3Do9CFheB8%3Dg6uT5TUm2gkvA%40mail.gmail.com

The new version of the streaming read API [1] is posted. I updated the
streaming read API changes patch (0001), using the streaming read API
in ANALYZE patch (0002) remains the same. This should make it easier
to review as it can be applied on top of master

[1]: https://www.postgresql.org/message-id/CA%2BhUKGJtLyxcAEvLhVUhgD4fMQkOu3PDaj8Qb9SR_UsmzgsBpQ%40mail.gmail.com

--
Regards,
Nazir Bilal Yavuz
Microsoft

On Tue, Mar 26, 2024 at 02:51:27PM +0300, Nazir Bilal Yavuz wrote:
> Hi,
> 
> On Wed, 28 Feb 2024 at 14:42, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
> >
> >
> > The new version of the streaming read API [1] is posted. I updated the
> > streaming read API changes patch (0001), using the streaming read API
> > in ANALYZE patch (0002) remains the same. This should make it easier
> > to review as it can be applied on top of master
> >
> >
> 
> The new version of the streaming read API is posted [1]. I rebased the
> patch on top of master and v9 of the streaming read API.
> 
> There is a minimal change in the 'using the streaming read API in ANALYZE
> patch (0002)', I changed STREAMING_READ_FULL to STREAMING_READ_MAINTENANCE
> to copy exactly the same behavior as before. Also, some benchmarking
> results:
> 
> I created a 22 GB table and set the size of shared buffers to 30GB, the
> rest is default.
> 
> ╔═══════════════════════════╦═════════════════════╦════════════╗
> ║                           ║  Avg Timings in ms  ║            ║
> ╠═══════════════════════════╬══════════╦══════════╬════════════╣
> ║                           ║  master  ║  patched ║ percentage ║
> ╠═══════════════════════════╬══════════╬══════════╬════════════╣
> ║     Both OS cache and     ║          ║          ║            ║
> ║  shared buffers are clear ║ 513.9247 ║ 463.1019 ║    %9.9    ║
> ╠═══════════════════════════╬══════════╬══════════╬════════════╣
> ║   OS cache is loaded but  ║          ║          ║            ║
> ║  shared buffers are clear ║ 423.1097 ║ 354.3277 ║    %16.3   ║
> ╠═══════════════════════════╬══════════╬══════════╬════════════╣
> ║ Shared buffers are loaded ║          ║          ║            ║
> ║                           ║  89.2846 ║  84.6952 ║    %5.1    ║
> ╚═══════════════════════════╩══════════╩══════════╩════════════╝
> 
> Any kind of feedback would be appreciated.

Thanks for working on this!

A general review comment: I noticed you have the old streaming read
(pgsr) naming still in a few places (including comments) -- so I would
just make sure and update everywhere when you rebase in Thomas' latest
version of the read stream API.

> From c7500cc1b9068ff0b704181442999cd8bed58658 Mon Sep 17 00:00:00 2001
> From: Nazir Bilal Yavuz <byavuz81@gmail.com>
> Date: Mon, 19 Feb 2024 14:30:47 +0300
> Subject: [PATCH v3 2/2] Use streaming read API in ANALYZE
>
> --- a/src/backend/commands/analyze.c
> +++ b/src/backend/commands/analyze.c
> @@ -1102,6 +1102,26 @@ examine_attribute(Relation onerel, int attnum, Node *index_expr)
>      return stats;
>  }
>  
> +/*
> + * Prefetch callback function to get next block number while using
> + * BlockSampling algorithm
> + */
> +static BlockNumber
> +pg_block_sampling_streaming_read_next(StreamingRead *stream,
> +                                      void *user_data,
> +                                      void *per_buffer_data)

I don't think you need the pg_ prefix

> +{
> +    BlockSamplerData *bs = user_data;
> +    BlockNumber *current_block = per_buffer_data;

Why can't you just do BufferGetBlockNumber() on the buffer returned from
the read stream API instead of allocating per_buffer_data for the block
number?

> +
> +    if (BlockSampler_HasMore(bs))
> +        *current_block = BlockSampler_Next(bs);
> +    else
> +        *current_block = InvalidBlockNumber;
> +
> +    return *current_block;


I think we'd like to keep the read stream code in heapam-specific code.
Instead of doing streaming_read_buffer_begin() here, you could put this
in heap_beginscan() or initscan() guarded by
    scan->rs_base.rs_flags & SO_TYPE_ANALYZE

same with streaming_read_buffer_end()/heap_endscan().

You'd also then need to save the reference to the read stream in the
HeapScanDescData.

> +    stream = streaming_read_buffer_begin(STREAMING_READ_MAINTENANCE,
> +                                         vac_strategy,
> +                                         BMR_REL(scan->rs_rd),
> +                                         MAIN_FORKNUM,
> +                                         pg_block_sampling_streaming_read_next,
> +                                         &bs,
> +                                         sizeof(BlockSamplerData));
>  
>      /* Outer loop over blocks to sample */

In fact, I think you could use this opportunity to get rid of the block
dependency in acquire_sample_rows() altogether.

Looking at the code now, it seems like you could just invoke
heapam_scan_analyze_next_block() (maybe rename it to
heapam_scan_analyze_next_buffer() or something) from
heapam_scan_analyze_next_tuple() and remove
table_scan_analyze_next_block() entirely.

Then table AMs can figure out how they want to return tuples from
table_scan_analyze_next_tuple().

If you do all this, note that you'll need to update the comments above
acquire_sample_rows() accordingly.

> -    while (BlockSampler_HasMore(&bs))
> +    while (nblocks)
>      {
>          bool        block_accepted;
> -        BlockNumber targblock = BlockSampler_Next(&bs);
> -#ifdef USE_PREFETCH
> -        BlockNumber prefetch_targblock = InvalidBlockNumber;
> -
> -        /*
> -         * Make sure that every time the main BlockSampler is moved forward
> -         * that our prefetch BlockSampler also gets moved forward, so that we
> -         * always stay out ahead.
> -         */
> -        if (prefetch_maximum && BlockSampler_HasMore(&prefetch_bs))
> -            prefetch_targblock = BlockSampler_Next(&prefetch_bs);
> -#endif
>  
>          vacuum_delay_point();
>  
> -        block_accepted = table_scan_analyze_next_block(scan, targblock, vac_strategy);
> +        block_accepted = table_scan_analyze_next_block(scan, stream);

- Melanie

Hi,

Thanks for the review!

On Wed, 27 Mar 2024 at 23:15, Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Tue, Mar 26, 2024 at 02:51:27PM +0300, Nazir Bilal Yavuz wrote:
> > Hi,
> >
> > On Wed, 28 Feb 2024 at 14:42, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
> > >
> > >
> > > The new version of the streaming read API [1] is posted. I updated the
> > > streaming read API changes patch (0001), using the streaming read API
> > > in ANALYZE patch (0002) remains the same. This should make it easier
> > > to review as it can be applied on top of master
> > >
> > >
> >
> > The new version of the streaming read API is posted [1]. I rebased the
> > patch on top of master and v9 of the streaming read API.
> >
> > There is a minimal change in the 'using the streaming read API in ANALYZE
> > patch (0002)', I changed STREAMING_READ_FULL to STREAMING_READ_MAINTENANCE
> > to copy exactly the same behavior as before. Also, some benchmarking
> > results:
> >
> > I created a 22 GB table and set the size of shared buffers to 30GB, the
> > rest is default.
> >
> > ╔═══════════════════════════╦═════════════════════╦════════════╗
> > ║                           ║  Avg Timings in ms  ║            ║
> > ╠═══════════════════════════╬══════════╦══════════╬════════════╣
> > ║                           ║  master  ║  patched ║ percentage ║
> > ╠═══════════════════════════╬══════════╬══════════╬════════════╣
> > ║     Both OS cache and     ║          ║          ║            ║
> > ║  shared buffers are clear ║ 513.9247 ║ 463.1019 ║    %9.9    ║
> > ╠═══════════════════════════╬══════════╬══════════╬════════════╣
> > ║   OS cache is loaded but  ║          ║          ║            ║
> > ║  shared buffers are clear ║ 423.1097 ║ 354.3277 ║    %16.3   ║
> > ╠═══════════════════════════╬══════════╬══════════╬════════════╣
> > ║ Shared buffers are loaded ║          ║          ║            ║
> > ║                           ║  89.2846 ║  84.6952 ║    %5.1    ║
> > ╚═══════════════════════════╩══════════╩══════════╩════════════╝
> >
> > Any kind of feedback would be appreciated.
>
> Thanks for working on this!
>
> A general review comment: I noticed you have the old streaming read
> (pgsr) naming still in a few places (including comments) -- so I would
> just make sure and update everywhere when you rebase in Thomas' latest
> version of the read stream API.

Done.

>
> > From c7500cc1b9068ff0b704181442999cd8bed58658 Mon Sep 17 00:00:00 2001
> > From: Nazir Bilal Yavuz <byavuz81@gmail.com>
> > Date: Mon, 19 Feb 2024 14:30:47 +0300
> > Subject: [PATCH v3 2/2] Use streaming read API in ANALYZE
> >
> > --- a/src/backend/commands/analyze.c
> > +++ b/src/backend/commands/analyze.c
> > @@ -1102,6 +1102,26 @@ examine_attribute(Relation onerel, int attnum, Node *index_expr)
> >       return stats;
> >  }
> >
> > +/*
> > + * Prefetch callback function to get next block number while using
> > + * BlockSampling algorithm
> > + */
> > +static BlockNumber
> > +pg_block_sampling_streaming_read_next(StreamingRead *stream,
> > +                                                                       void *user_data,
> > +                                                                       void *per_buffer_data)
>
> I don't think you need the pg_ prefix

Done.

>
> > +{
> > +     BlockSamplerData *bs = user_data;
> > +     BlockNumber *current_block = per_buffer_data;
>
> Why can't you just do BufferGetBlockNumber() on the buffer returned from
> the read stream API instead of allocating per_buffer_data for the block
> number?

Done.

>
> > +
> > +     if (BlockSampler_HasMore(bs))
> > +             *current_block = BlockSampler_Next(bs);
> > +     else
> > +             *current_block = InvalidBlockNumber;
> > +
> > +     return *current_block;
>
>
> I think we'd like to keep the read stream code in heapam-specific code.
> Instead of doing streaming_read_buffer_begin() here, you could put this
> in heap_beginscan() or initscan() guarded by
>         scan->rs_base.rs_flags & SO_TYPE_ANALYZE

In the recent changes [1], heapam_scan_analyze_next_[block | tuple]
are removed from tableam. They are directly called from
heapam-specific code now. So, IMO, no need to do this now.

v4 is rebased on top of v14 streaming read API changes.

[1] 27bc1772fc814946918a5ac8ccb9b5c5ad0380aa

-- 
Regards,
Nazir Bilal Yavuz
Microsoft

On Tue, Apr 2, 2024 at 9:24 AM Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
[..]
> v4 is rebased on top of v14 streaming read API changes.

Hi Nazir, so with streaming API committed, I gave a try to this patch.
With autovacuum=off and 30GB table on NVMe (with standard readahead of
256kb and ext4, Debian 12, kernel 6.1.0, shared_buffers = 128MB
default) created using: create table t as select repeat('a', 100) || i
|| repeat('b', 500) as filler from generate_series(1, 45000000) as i;

on master, effect of mainteance_io_concurency [default 10] is like
that (when resetting the fs cache after each ANALYZE):

    m_io_c = 0:
    Time: 3137.914 ms (00:03.138)
    Time: 3094.540 ms (00:03.095)
    Time: 3452.513 ms (00:03.453)

    m_io_c = 1:
    Time: 2972.751 ms (00:02.973)
    Time: 2939.551 ms (00:02.940)
    Time: 2904.428 ms (00:02.904)

    m_io_c = 2:
    Time: 1580.260 ms (00:01.580)
    Time: 1572.132 ms (00:01.572)
    Time: 1558.334 ms (00:01.558)

    m_io_c = 4:
    Time: 938.304 ms
    Time: 931.772 ms
    Time: 920.044 ms

    m_io_c = 8:
    Time: 666.025 ms
    Time: 660.241 ms
    Time: 648.848 ms

    m_io_c = 16:
    Time: 542.450 ms
    Time: 561.155 ms
    Time: 539.683 ms

    m_io_c = 32:
    Time: 538.487 ms
    Time: 541.705 ms
    Time: 538.101 ms

with patch applied:

    m_io_c = 0:
    Time: 3106.469 ms (00:03.106)
    Time: 3140.343 ms (00:03.140)
    Time: 3044.133 ms (00:03.044)

    m_io_c = 1:
    Time: 2959.817 ms (00:02.960)
    Time: 2920.265 ms (00:02.920)
    Time: 2911.745 ms (00:02.912)

    m_io_c = 2:
    Time: 1581.912 ms (00:01.582)
    Time: 1561.444 ms (00:01.561)
    Time: 1558.251 ms (00:01.558)

    m_io_c = 4:
    Time: 908.116 ms
    Time: 901.245 ms
    Time: 901.071 ms

    m_io_c = 8:
    Time: 619.870 ms
    Time: 620.327 ms
    Time: 614.266 ms

    m_io_c = 16:
    Time: 529.885 ms
    Time: 526.958 ms
    Time: 528.474 ms

    m_io_c = 32:
    Time: 521.185 ms
    Time: 520.713 ms
    Time: 517.729 ms

No difference to me, which seems to be good. I've double checked and
patch used the new way

acquire_sample_rows -> heapam_scan_analyze_next_block ->
ReadBufferExtended -> ReadBuffer_common (inlined) -> WaitReadBuffers
-> mdreadv -> FileReadV -> pg_preadv (inlined)
acquire_sample_rows -> heapam_scan_analyze_next_block ->
ReadBufferExtended -> ReadBuffer_common (inlined) -> StartReadBuffer
-> ...

I gave also io_combine_limit to 32 (max, 256kB) a try and got those
slightly better results:

[..]
m_io_c = 16:
Time: 494.599 ms
Time: 496.345 ms
Time: 973.500 ms

m_io_c = 32:
Time: 461.031 ms
Time: 449.037 ms
Time: 443.375 ms

and that (last one) apparently was able to push it to ~50-60k still
random IOPS range, the rareq-sz was still ~8 (9.9) kB as analyze was
still reading random , so I assume no merging was done:

Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz
w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s
drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
nvme0n1       61212.00    591.82     0.00   0.00    0.10     9.90
2.00      0.02     0.00   0.00    0.00    12.00    0.00      0.00
0.00   0.00    0.00     0.00    0.00    0.00    6.28  85.20

So in short it looks good to me.

-Jakub Wartak.

Hi,

On Tue, 2 Apr 2024 at 10:23, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
>
> v4 is rebased on top of v14 streaming read API changes.

Streaming API has been committed but the committed version has a minor
change, the read_stream_begin_relation function takes Relation instead
of BufferManagerRelation now. So, here is a v5 which addresses this
change.

-- 
Regards,
Nazir Bilal Yavuz
Microsoft

On 03/04/2024 13:31, Nazir Bilal Yavuz wrote:
> Streaming API has been committed but the committed version has a minor
> change, the read_stream_begin_relation function takes Relation instead
> of BufferManagerRelation now. So, here is a v5 which addresses this
> change.

I'm getting a repeatable segfault / assertion failure with this:

postgres=# CREATE TABLE tengiga (i int, filler text) with (fillfactor=10);
CREATE TABLE
postgres=# insert into tengiga select g, repeat('x', 900) from 
generate_series(1, 1400000) g;
INSERT 0 1400000
postgres=# set default_statistics_target = 10; ANALYZE tengiga;
SET
ANALYZE
postgres=# set default_statistics_target = 100; ANALYZE tengiga;
SET
ANALYZE
postgres=# set default_statistics_target =1000; ANALYZE tengiga;
SET
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.

TRAP: failed Assert("BufferIsValid(hscan->rs_cbuf)"), File: 
"heapam_handler.c", Line: 1079, PID: 262232
postgres: heikki postgres [local] 
ANALYZE(ExceptionalCondition+0xa8)[0x56488a0de9d8]
postgres: heikki postgres [local] 
ANALYZE(heapam_scan_analyze_next_block+0x63)[0x5648899ece34]
postgres: heikki postgres [local] ANALYZE(+0x2d3f34)[0x564889b6af34]
postgres: heikki postgres [local] ANALYZE(+0x2d2a3a)[0x564889b69a3a]
postgres: heikki postgres [local] ANALYZE(analyze_rel+0x33e)[0x564889b68fa9]
postgres: heikki postgres [local] ANALYZE(vacuum+0x4b3)[0x564889c2dcc0]
postgres: heikki postgres [local] ANALYZE(ExecVacuum+0xd6f)[0x564889c2d7fe]
postgres: heikki postgres [local] 
ANALYZE(standard_ProcessUtility+0x901)[0x564889f0b8b9]
postgres: heikki postgres [local] 
ANALYZE(ProcessUtility+0x136)[0x564889f0afb1]
postgres: heikki postgres [local] ANALYZE(+0x6728c8)[0x564889f098c8]
postgres: heikki postgres [local] ANALYZE(+0x672b3b)[0x564889f09b3b]
postgres: heikki postgres [local] ANALYZE(PortalRun+0x320)[0x564889f09015]
postgres: heikki postgres [local] ANALYZE(+0x66b2c6)[0x564889f022c6]
postgres: heikki postgres [local] 
ANALYZE(PostgresMain+0x80c)[0x564889f06fd7]
postgres: heikki postgres [local] ANALYZE(+0x667876)[0x564889efe876]
postgres: heikki postgres [local] 
ANALYZE(postmaster_child_launch+0xe6)[0x564889e1f4b3]
postgres: heikki postgres [local] ANALYZE(+0x58e68e)[0x564889e2568e]
postgres: heikki postgres [local] ANALYZE(+0x58b7f0)[0x564889e227f0]
postgres: heikki postgres [local] 
ANALYZE(PostmasterMain+0x152b)[0x564889e2214d]
postgres: heikki postgres [local] ANALYZE(+0x4444b4)[0x564889cdb4b4]
/lib/x86_64-linux-gnu/libc.so.6(+0x2724a)[0x7f7d83b6724a]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7f7d83b67305]
postgres: heikki postgres [local] ANALYZE(_start+0x21)[0x564889971a61]
2024-04-03 20:15:49.157 EEST [262101] LOG:  server process (PID 262232) 
was terminated by signal 6: Aborted

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Hi Jakub,

Thank you for looking into this and doing a performance analysis.

On Wed, 3 Apr 2024 at 11:42, Jakub Wartak <jakub.wartak@enterprisedb.com> wrote:
>
> On Tue, Apr 2, 2024 at 9:24 AM Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
> [..]
> > v4 is rebased on top of v14 streaming read API changes.
>
> Hi Nazir, so with streaming API committed, I gave a try to this patch.
> With autovacuum=off and 30GB table on NVMe (with standard readahead of
> 256kb and ext4, Debian 12, kernel 6.1.0, shared_buffers = 128MB
> default) created using: create table t as select repeat('a', 100) || i
> || repeat('b', 500) as filler from generate_series(1, 45000000) as i;
>
> on master, effect of mainteance_io_concurency [default 10] is like
> that (when resetting the fs cache after each ANALYZE):
>
>     m_io_c = 0:
>     Time: 3137.914 ms (00:03.138)
>     Time: 3094.540 ms (00:03.095)
>     Time: 3452.513 ms (00:03.453)
>
>     m_io_c = 1:
>     Time: 2972.751 ms (00:02.973)
>     Time: 2939.551 ms (00:02.940)
>     Time: 2904.428 ms (00:02.904)
>
>     m_io_c = 2:
>     Time: 1580.260 ms (00:01.580)
>     Time: 1572.132 ms (00:01.572)
>     Time: 1558.334 ms (00:01.558)
>
>     m_io_c = 4:
>     Time: 938.304 ms
>     Time: 931.772 ms
>     Time: 920.044 ms
>
>     m_io_c = 8:
>     Time: 666.025 ms
>     Time: 660.241 ms
>     Time: 648.848 ms
>
>     m_io_c = 16:
>     Time: 542.450 ms
>     Time: 561.155 ms
>     Time: 539.683 ms
>
>     m_io_c = 32:
>     Time: 538.487 ms
>     Time: 541.705 ms
>     Time: 538.101 ms
>
> with patch applied:
>
>     m_io_c = 0:
>     Time: 3106.469 ms (00:03.106)
>     Time: 3140.343 ms (00:03.140)
>     Time: 3044.133 ms (00:03.044)
>
>     m_io_c = 1:
>     Time: 2959.817 ms (00:02.960)
>     Time: 2920.265 ms (00:02.920)
>     Time: 2911.745 ms (00:02.912)
>
>     m_io_c = 2:
>     Time: 1581.912 ms (00:01.582)
>     Time: 1561.444 ms (00:01.561)
>     Time: 1558.251 ms (00:01.558)
>
>     m_io_c = 4:
>     Time: 908.116 ms
>     Time: 901.245 ms
>     Time: 901.071 ms
>
>     m_io_c = 8:
>     Time: 619.870 ms
>     Time: 620.327 ms
>     Time: 614.266 ms
>
>     m_io_c = 16:
>     Time: 529.885 ms
>     Time: 526.958 ms
>     Time: 528.474 ms
>
>     m_io_c = 32:
>     Time: 521.185 ms
>     Time: 520.713 ms
>     Time: 517.729 ms
>
> No difference to me, which seems to be good. I've double checked and
> patch used the new way
>
> acquire_sample_rows -> heapam_scan_analyze_next_block ->
> ReadBufferExtended -> ReadBuffer_common (inlined) -> WaitReadBuffers
> -> mdreadv -> FileReadV -> pg_preadv (inlined)
> acquire_sample_rows -> heapam_scan_analyze_next_block ->
> ReadBufferExtended -> ReadBuffer_common (inlined) -> StartReadBuffer
> -> ...
>
> I gave also io_combine_limit to 32 (max, 256kB) a try and got those
> slightly better results:
>
> [..]
> m_io_c = 16:
> Time: 494.599 ms
> Time: 496.345 ms
> Time: 973.500 ms
>
> m_io_c = 32:
> Time: 461.031 ms
> Time: 449.037 ms
> Time: 443.375 ms
>
> and that (last one) apparently was able to push it to ~50-60k still
> random IOPS range, the rareq-sz was still ~8 (9.9) kB as analyze was
> still reading random , so I assume no merging was done:
>
> Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz
> w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s
> drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
> nvme0n1       61212.00    591.82     0.00   0.00    0.10     9.90
> 2.00      0.02     0.00   0.00    0.00    12.00    0.00      0.00
> 0.00   0.00    0.00     0.00    0.00    0.00    6.28  85.20
>
> So in short it looks good to me.

My results are similar to yours, also I realized a bug while working
on your benchmarking cases. I will share the cause and the fix soon.

--
Regards,
Nazir Bilal Yavuz
Microsoft

Hi,

Thank you for looking into this!

On Wed, 3 Apr 2024 at 20:17, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>
> On 03/04/2024 13:31, Nazir Bilal Yavuz wrote:
> > Streaming API has been committed but the committed version has a minor
> > change, the read_stream_begin_relation function takes Relation instead
> > of BufferManagerRelation now. So, here is a v5 which addresses this
> > change.
>
> I'm getting a repeatable segfault / assertion failure with this:
>
> postgres=# CREATE TABLE tengiga (i int, filler text) with (fillfactor=10);
> CREATE TABLE
> postgres=# insert into tengiga select g, repeat('x', 900) from
> generate_series(1, 1400000) g;
> INSERT 0 1400000
> postgres=# set default_statistics_target = 10; ANALYZE tengiga;
> SET
> ANALYZE
> postgres=# set default_statistics_target = 100; ANALYZE tengiga;
> SET
> ANALYZE
> postgres=# set default_statistics_target =1000; ANALYZE tengiga;
> SET
> server closed the connection unexpectedly
>         This probably means the server terminated abnormally
>         before or while processing the request.
>
> TRAP: failed Assert("BufferIsValid(hscan->rs_cbuf)"), File:
> "heapam_handler.c", Line: 1079, PID: 262232
> postgres: heikki postgres [local]
> ANALYZE(ExceptionalCondition+0xa8)[0x56488a0de9d8]
> postgres: heikki postgres [local]
> ANALYZE(heapam_scan_analyze_next_block+0x63)[0x5648899ece34]
> postgres: heikki postgres [local] ANALYZE(+0x2d3f34)[0x564889b6af34]
> postgres: heikki postgres [local] ANALYZE(+0x2d2a3a)[0x564889b69a3a]
> postgres: heikki postgres [local] ANALYZE(analyze_rel+0x33e)[0x564889b68fa9]
> postgres: heikki postgres [local] ANALYZE(vacuum+0x4b3)[0x564889c2dcc0]
> postgres: heikki postgres [local] ANALYZE(ExecVacuum+0xd6f)[0x564889c2d7fe]
> postgres: heikki postgres [local]
> ANALYZE(standard_ProcessUtility+0x901)[0x564889f0b8b9]
> postgres: heikki postgres [local]
> ANALYZE(ProcessUtility+0x136)[0x564889f0afb1]
> postgres: heikki postgres [local] ANALYZE(+0x6728c8)[0x564889f098c8]
> postgres: heikki postgres [local] ANALYZE(+0x672b3b)[0x564889f09b3b]
> postgres: heikki postgres [local] ANALYZE(PortalRun+0x320)[0x564889f09015]
> postgres: heikki postgres [local] ANALYZE(+0x66b2c6)[0x564889f022c6]
> postgres: heikki postgres [local]
> ANALYZE(PostgresMain+0x80c)[0x564889f06fd7]
> postgres: heikki postgres [local] ANALYZE(+0x667876)[0x564889efe876]
> postgres: heikki postgres [local]
> ANALYZE(postmaster_child_launch+0xe6)[0x564889e1f4b3]
> postgres: heikki postgres [local] ANALYZE(+0x58e68e)[0x564889e2568e]
> postgres: heikki postgres [local] ANALYZE(+0x58b7f0)[0x564889e227f0]
> postgres: heikki postgres [local]
> ANALYZE(PostmasterMain+0x152b)[0x564889e2214d]
> postgres: heikki postgres [local] ANALYZE(+0x4444b4)[0x564889cdb4b4]
> /lib/x86_64-linux-gnu/libc.so.6(+0x2724a)[0x7f7d83b6724a]
> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7f7d83b67305]
> postgres: heikki postgres [local] ANALYZE(_start+0x21)[0x564889971a61]
> 2024-04-03 20:15:49.157 EEST [262101] LOG:  server process (PID 262232)
> was terminated by signal 6: Aborted

I realized the same error while working on Jakub's benchmarking results.

Cause: I was using the nblocks variable to check how many blocks will
be returned from the streaming API. But I realized that sometimes the
number returned from BlockSampler_Init() is not equal to the number of
blocks that BlockSampler_Next() will return as BlockSampling algorithm
decides how many blocks to return on the fly by using some random
seeds.

There are a couple of solutions I thought of:

1- Use BlockSampler_HasMore() instead of nblocks in the main loop in
the acquire_sample_rows():

Streaming API uses this function to prefetch block numbers.
BlockSampler_HasMore() will reach to the end first as it is used while
prefetching, so it will start to return false while there are still
buffers to return from the streaming API. That will cause some buffers
at the end to not be processed.

2- Expose something (function, variable etc.) from the streaming API
to understand if the read is finished and there is no buffer to
return:

I think this works but I am not sure if the streaming API allows
something like that.

3- Check every buffer returned from the streaming API, if it is
invalid stop the main loop in the acquire_sample_rows():

This solves the problem but there will be two if checks for each
buffer returned,
- in heapam_scan_analyze_next_block() to check if the returned buffer is invalid
- to break main loop in acquire_sample_rows() if
heapam_scan_analyze_next_block() returns false
One of the if cases can be bypassed by moving
heapam_scan_analyze_next_block()'s code to the main loop in the
acquire_sample_rows().

I implemented the third solution, here is v6.

-- 
Regards,
Nazir Bilal Yavuz
Microsoft

On Wed, Apr 03, 2024 at 10:25:01PM +0300, Nazir Bilal Yavuz wrote:
>
> I realized the same error while working on Jakub's benchmarking results.
> 
> Cause: I was using the nblocks variable to check how many blocks will
> be returned from the streaming API. But I realized that sometimes the
> number returned from BlockSampler_Init() is not equal to the number of
> blocks that BlockSampler_Next() will return as BlockSampling algorithm
> decides how many blocks to return on the fly by using some random
> seeds.
> 
> There are a couple of solutions I thought of:
> 
> 1- Use BlockSampler_HasMore() instead of nblocks in the main loop in
> the acquire_sample_rows():
> 
> Streaming API uses this function to prefetch block numbers.
> BlockSampler_HasMore() will reach to the end first as it is used while
> prefetching, so it will start to return false while there are still
> buffers to return from the streaming API. That will cause some buffers
> at the end to not be processed.
> 
> 2- Expose something (function, variable etc.) from the streaming API
> to understand if the read is finished and there is no buffer to
> return:
> 
> I think this works but I am not sure if the streaming API allows
> something like that.
> 
> 3- Check every buffer returned from the streaming API, if it is
> invalid stop the main loop in the acquire_sample_rows():
> 
> This solves the problem but there will be two if checks for each
> buffer returned,
> - in heapam_scan_analyze_next_block() to check if the returned buffer is invalid
> - to break main loop in acquire_sample_rows() if
> heapam_scan_analyze_next_block() returns false
> One of the if cases can be bypassed by moving
> heapam_scan_analyze_next_block()'s code to the main loop in the
> acquire_sample_rows().
> 
> I implemented the third solution, here is v6.

I've reviewed the patches inline below and attached a patch that has
some of my ideas on top of your patch.

> From 8d396a42186325f920d5a05e7092d8e1b66f3cdf Mon Sep 17 00:00:00 2001
> From: Nazir Bilal Yavuz <byavuz81@gmail.com>
> Date: Wed, 3 Apr 2024 15:14:15 +0300
> Subject: [PATCH v6] Use streaming read API in ANALYZE
> 
> ANALYZE command gets random tuples using BlockSampler algorithm. Use
> streaming reads to get these tuples by using BlockSampler algorithm in
> streaming read API prefetch logic.
> ---
>  src/include/access/heapam.h              |  6 +-
>  src/backend/access/heap/heapam_handler.c | 22 +++---
>  src/backend/commands/analyze.c           | 85 ++++++++----------------
>  3 files changed, 42 insertions(+), 71 deletions(-)
> 
> diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
> index a307fb5f245..633caee9d95 100644
> --- a/src/include/access/heapam.h
> +++ b/src/include/access/heapam.h
> @@ -25,6 +25,7 @@
>  #include "storage/bufpage.h"
>  #include "storage/dsm.h"
>  #include "storage/lockdefs.h"
> +#include "storage/read_stream.h"
>  #include "storage/shm_toc.h"
>  #include "utils/relcache.h"
>  #include "utils/snapshot.h"
> @@ -388,9 +389,8 @@ extern bool HeapTupleIsSurelyDead(HeapTuple htup,
>                                    struct GlobalVisState *vistest);
>  
>  /* in heap/heapam_handler.c*/
> -extern void heapam_scan_analyze_next_block(TableScanDesc scan,
> -                                           BlockNumber blockno,
> -                                           BufferAccessStrategy bstrategy);
> +extern bool heapam_scan_analyze_next_block(TableScanDesc scan,
> +                                           ReadStream *stream);
>  extern bool heapam_scan_analyze_next_tuple(TableScanDesc scan,
>                                             TransactionId OldestXmin,
>                                             double *liverows, double *deadrows,
> diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
> index 0952d4a98eb..d83fbbe6af3 100644
> --- a/src/backend/access/heap/heapam_handler.c
> +++ b/src/backend/access/heap/heapam_handler.c
> @@ -1054,16 +1054,16 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
>  }
>  
>  /*
> - * Prepare to analyze block `blockno` of `scan`.  The scan has been started
> - * with SO_TYPE_ANALYZE option.
> + * Prepare to analyze block returned from streaming object.  If the block returned
> + * from streaming object is valid, true is returned; otherwise false is returned.
> + * The scan has been started with SO_TYPE_ANALYZE option.
>   *
>   * This routine holds a buffer pin and lock on the heap page.  They are held
>   * until heapam_scan_analyze_next_tuple() returns false.  That is until all the
>   * items of the heap page are analyzed.
>   */
> -void
> -heapam_scan_analyze_next_block(TableScanDesc scan, BlockNumber blockno,
> -                               BufferAccessStrategy bstrategy)
> +bool
> +heapam_scan_analyze_next_block(TableScanDesc scan, ReadStream *stream)
>  {
>      HeapScanDesc hscan = (HeapScanDesc) scan;
>  
> @@ -1076,11 +1076,15 @@ heapam_scan_analyze_next_block(TableScanDesc scan, BlockNumber blockno,
>       * doing much work per tuple, the extra lock traffic is probably better
>       * avoided.

Personally I think heapam_scan_analyze_next_block() should be inlined.
It only has a few lines. I would find it clearer inline. At the least,
there is no reason for it (or heapam_scan_analyze_next_tuple()) to take
a TableScanDesc instead of a HeapScanDesc.

>       */
> -    hscan->rs_cblock = blockno;
> -    hscan->rs_cindex = FirstOffsetNumber;
> -    hscan->rs_cbuf = ReadBufferExtended(scan->rs_rd, MAIN_FORKNUM,
> -                                        blockno, RBM_NORMAL, bstrategy);
> +    hscan->rs_cbuf = read_stream_next_buffer(stream, NULL);
> +    if (hscan->rs_cbuf == InvalidBuffer)
> +        return false;
> +
>      LockBuffer(hscan->rs_cbuf, BUFFER_LOCK_SHARE);
> +
> +    hscan->rs_cblock = BufferGetBlockNumber(hscan->rs_cbuf);
> +    hscan->rs_cindex = FirstOffsetNumber;
> +    return true;
>  }

>  /*
> diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
> index 2fb39f3ede1..764520d5aa2 100644
> --- a/src/backend/commands/analyze.c
> +++ b/src/backend/commands/analyze.c
> @@ -1102,6 +1102,20 @@ examine_attribute(Relation onerel, int attnum, Node *index_expr)
>      return stats;
>  }
>  
> +/*
> + * Prefetch callback function to get next block number while using
> + * BlockSampling algorithm
> + */
> +static BlockNumber
> +block_sampling_streaming_read_next(ReadStream *stream,
> +                                   void *user_data,
> +                                   void *per_buffer_data)
> +{
> +    BlockSamplerData *bs = user_data;
> +
> +    return BlockSampler_HasMore(bs) ? BlockSampler_Next(bs) : InvalidBlockNumber;

I don't see the point of BlockSampler_HasMore() anymore. I removed it in
the attached and made BlockSampler_Next() return InvalidBlockNumber
under the same conditions. Is there a reason not to do this? There
aren't other callers. If the BlockSampler_Next() wasn't part of an API,
we could just make it the streaming read callback, but that might be
weird as it is now.

That and my other ideas in attached. Let me know what you think.

- Melanie

Hi,

On Wed, 3 Apr 2024 at 23:44, Melanie Plageman <melanieplageman@gmail.com> wrote:
>
>
> I've reviewed the patches inline below and attached a patch that has
> some of my ideas on top of your patch.

Thank you!

>
> > From 8d396a42186325f920d5a05e7092d8e1b66f3cdf Mon Sep 17 00:00:00 2001
> > From: Nazir Bilal Yavuz <byavuz81@gmail.com>
> > Date: Wed, 3 Apr 2024 15:14:15 +0300
> > Subject: [PATCH v6] Use streaming read API in ANALYZE
> >
> > ANALYZE command gets random tuples using BlockSampler algorithm. Use
> > streaming reads to get these tuples by using BlockSampler algorithm in
> > streaming read API prefetch logic.
> > ---
> >  src/include/access/heapam.h              |  6 +-
> >  src/backend/access/heap/heapam_handler.c | 22 +++---
> >  src/backend/commands/analyze.c           | 85 ++++++++----------------
> >  3 files changed, 42 insertions(+), 71 deletions(-)
> >
> > diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
> > index a307fb5f245..633caee9d95 100644
> > --- a/src/include/access/heapam.h
> > +++ b/src/include/access/heapam.h
> > @@ -25,6 +25,7 @@
> >  #include "storage/bufpage.h"
> >  #include "storage/dsm.h"
> >  #include "storage/lockdefs.h"
> > +#include "storage/read_stream.h"
> >  #include "storage/shm_toc.h"
> >  #include "utils/relcache.h"
> >  #include "utils/snapshot.h"
> > @@ -388,9 +389,8 @@ extern bool HeapTupleIsSurelyDead(HeapTuple htup,
> >                                                                 struct GlobalVisState *vistest);
> >
> >  /* in heap/heapam_handler.c*/
> > -extern void heapam_scan_analyze_next_block(TableScanDesc scan,
> > -                                                                                BlockNumber blockno,
> > -                                                                                BufferAccessStrategy bstrategy);
> > +extern bool heapam_scan_analyze_next_block(TableScanDesc scan,
> > +                                                                                ReadStream *stream);
> >  extern bool heapam_scan_analyze_next_tuple(TableScanDesc scan,
> >                                                                                  TransactionId OldestXmin,
> >                                                                                  double *liverows, double
*deadrows,
> > diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
> > index 0952d4a98eb..d83fbbe6af3 100644
> > --- a/src/backend/access/heap/heapam_handler.c
> > +++ b/src/backend/access/heap/heapam_handler.c
> > @@ -1054,16 +1054,16 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
> >  }
> >
> >  /*
> > - * Prepare to analyze block `blockno` of `scan`.  The scan has been started
> > - * with SO_TYPE_ANALYZE option.
> > + * Prepare to analyze block returned from streaming object.  If the block returned
> > + * from streaming object is valid, true is returned; otherwise false is returned.
> > + * The scan has been started with SO_TYPE_ANALYZE option.
> >   *
> >   * This routine holds a buffer pin and lock on the heap page.  They are held
> >   * until heapam_scan_analyze_next_tuple() returns false.  That is until all the
> >   * items of the heap page are analyzed.
> >   */
> > -void
> > -heapam_scan_analyze_next_block(TableScanDesc scan, BlockNumber blockno,
> > -                                                        BufferAccessStrategy bstrategy)
> > +bool
> > +heapam_scan_analyze_next_block(TableScanDesc scan, ReadStream *stream)
> >  {
> >       HeapScanDesc hscan = (HeapScanDesc) scan;
> >
> > @@ -1076,11 +1076,15 @@ heapam_scan_analyze_next_block(TableScanDesc scan, BlockNumber blockno,
> >        * doing much work per tuple, the extra lock traffic is probably better
> >        * avoided.
>
> Personally I think heapam_scan_analyze_next_block() should be inlined.
> It only has a few lines. I would find it clearer inline. At the least,
> there is no reason for it (or heapam_scan_analyze_next_tuple()) to take
> a TableScanDesc instead of a HeapScanDesc.

I agree.

>
> >        */
> > -     hscan->rs_cblock = blockno;
> > -     hscan->rs_cindex = FirstOffsetNumber;
> > -     hscan->rs_cbuf = ReadBufferExtended(scan->rs_rd, MAIN_FORKNUM,
> > -                                                                             blockno, RBM_NORMAL, bstrategy);
> > +     hscan->rs_cbuf = read_stream_next_buffer(stream, NULL);
> > +     if (hscan->rs_cbuf == InvalidBuffer)
> > +             return false;
> > +
> >       LockBuffer(hscan->rs_cbuf, BUFFER_LOCK_SHARE);
> > +
> > +     hscan->rs_cblock = BufferGetBlockNumber(hscan->rs_cbuf);
> > +     hscan->rs_cindex = FirstOffsetNumber;
> > +     return true;
> >  }
>
> >  /*
> > diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
> > index 2fb39f3ede1..764520d5aa2 100644
> > --- a/src/backend/commands/analyze.c
> > +++ b/src/backend/commands/analyze.c
> > @@ -1102,6 +1102,20 @@ examine_attribute(Relation onerel, int attnum, Node *index_expr)
> >       return stats;
> >  }
> >
> > +/*
> > + * Prefetch callback function to get next block number while using
> > + * BlockSampling algorithm
> > + */
> > +static BlockNumber
> > +block_sampling_streaming_read_next(ReadStream *stream,
> > +                                                                void *user_data,
> > +                                                                void *per_buffer_data)
> > +{
> > +     BlockSamplerData *bs = user_data;
> > +
> > +     return BlockSampler_HasMore(bs) ? BlockSampler_Next(bs) : InvalidBlockNumber;
>
> I don't see the point of BlockSampler_HasMore() anymore. I removed it in
> the attached and made BlockSampler_Next() return InvalidBlockNumber
> under the same conditions. Is there a reason not to do this? There
> aren't other callers. If the BlockSampler_Next() wasn't part of an API,
> we could just make it the streaming read callback, but that might be
> weird as it is now.

I agree. There is no reason to have BlockSampler_HasMore() after
streaming read API changes.

> That and my other ideas in attached. Let me know what you think.

I agree with your changes but I am not sure if others agree with all
the changes you have proposed. So, I didn't merge 0001 and your ideas
yet, instead I wrote a commit message, added some comments, changed ->
'if (bs->t >= bs->N || bs->m >= bs->n)' to 'if (K <= 0 || k <= 0)' and
attached it as 0002.

--
Regards,
Nazir Bilal Yavuz
Microsoft

On Thu, Apr 04, 2024 at 02:03:30PM +0300, Nazir Bilal Yavuz wrote:
> 
> On Wed, 3 Apr 2024 at 23:44, Melanie Plageman <melanieplageman@gmail.com> wrote:
> >
> > I don't see the point of BlockSampler_HasMore() anymore. I removed it in
> > the attached and made BlockSampler_Next() return InvalidBlockNumber
> > under the same conditions. Is there a reason not to do this? There
> > aren't other callers. If the BlockSampler_Next() wasn't part of an API,
> > we could just make it the streaming read callback, but that might be
> > weird as it is now.
> 
> I agree. There is no reason to have BlockSampler_HasMore() after
> streaming read API changes.
> 
> > That and my other ideas in attached. Let me know what you think.
> 
> I agree with your changes but I am not sure if others agree with all
> the changes you have proposed. So, I didn't merge 0001 and your ideas
> yet, instead I wrote a commit message, added some comments, changed ->
> 'if (bs->t >= bs->N || bs->m >= bs->n)' to 'if (K <= 0 || k <= 0)' and
> attached it as 0002.

I couldn't quite let go of those changes to acquire_sample_rows(), so
attached v9 0001 implements them as a preliminary patch before your
analyze streaming read user. I inlined heapam_scan_analyze_next_block()
entirely and made heapam_scan_analyze_next_tuple() a static function in
commands/analyze.c (and tweaked the name).

I made a few tweaks to your patch since it is on top of those changes
instead of preceding them. Then 0003 is removing BlockSampler_HasMore()
since it doesn't make sense to remove it before the streaming read user
was added.

- Melanie

On Sun, Apr 7, 2024 at 3:57 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Thu, Apr 04, 2024 at 02:03:30PM +0300, Nazir Bilal Yavuz wrote:
> >
> > On Wed, 3 Apr 2024 at 23:44, Melanie Plageman <melanieplageman@gmail.com> wrote:
> > >
> > > I don't see the point of BlockSampler_HasMore() anymore. I removed it in
> > > the attached and made BlockSampler_Next() return InvalidBlockNumber
> > > under the same conditions. Is there a reason not to do this? There
> > > aren't other callers. If the BlockSampler_Next() wasn't part of an API,
> > > we could just make it the streaming read callback, but that might be
> > > weird as it is now.
> >
> > I agree. There is no reason to have BlockSampler_HasMore() after
> > streaming read API changes.
> >
> > > That and my other ideas in attached. Let me know what you think.
> >
> > I agree with your changes but I am not sure if others agree with all
> > the changes you have proposed. So, I didn't merge 0001 and your ideas
> > yet, instead I wrote a commit message, added some comments, changed ->
> > 'if (bs->t >= bs->N || bs->m >= bs->n)' to 'if (K <= 0 || k <= 0)' and
> > attached it as 0002.
>
> I couldn't quite let go of those changes to acquire_sample_rows(), so
> attached v9 0001 implements them as a preliminary patch before your
> analyze streaming read user. I inlined heapam_scan_analyze_next_block()
> entirely and made heapam_scan_analyze_next_tuple() a static function in
> commands/analyze.c (and tweaked the name).
>
> I made a few tweaks to your patch since it is on top of those changes
> instead of preceding them. Then 0003 is removing BlockSampler_HasMore()
> since it doesn't make sense to remove it before the streaming read user
> was added.

I realized there were a few outdated comments. Fixed in attached v10.

- Melanie

Hi,

On 2024-04-07 16:59:26 -0400, Melanie Plageman wrote:
> From 1dc2343661f3edb3b1bc4307afb0e956397eb76c Mon Sep 17 00:00:00 2001
> From: Melanie Plageman <melanieplageman@gmail.com>
> Date: Sun, 7 Apr 2024 14:55:22 -0400
> Subject: [PATCH v10 1/3] Make heapam_scan_analyze_next_[tuple|block] static.
> 
> 27bc1772fc81 removed the table AM callbacks scan_analyze_next_block and
> scan_analzye_next_tuple -- leaving their heap AM implementations only
> called by acquire_sample_rows().

Ugh, I don't think 27bc1772fc81 makes much sense. But that's unrelated to this
thread.  I did raise that separately
https://www.postgresql.org/message-id/20240407214001.jgpg5q3yv33ve6y3%40awork3.anarazel.de

Unless I seriously missed something, I see no alternative to reverting that
commit.


> @@ -1206,11 +1357,13 @@ acquire_sample_rows(Relation onerel, int elevel,
>                  break;
>  
>              prefetch_block = BlockSampler_Next(&prefetch_bs);
> -            PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, prefetch_block);
> +            PrefetchBuffer(scan->rs_base.rs_rd, MAIN_FORKNUM, prefetch_block);
>          }
>      }
>  #endif
>  
> +    scan->rs_cbuf = InvalidBuffer;
> +
>      /* Outer loop over blocks to sample */
>      while (BlockSampler_HasMore(&bs))
>      {

I don't think it's good to move a lot of code *and* change how it is
structured in the same commit. Makes it much harder to actually see changes /
makes git blame harder to use / etc.



> From 90d115c2401567be65bcf64393a6d3b39286779e Mon Sep 17 00:00:00 2001
> From: Melanie Plageman <melanieplageman@gmail.com>
> Date: Sun, 7 Apr 2024 15:28:32 -0400
> Subject: [PATCH v10 2/3] Use streaming read API in ANALYZE
>
> The ANALYZE command prefetches and reads sample blocks chosen by a
> BlockSampler algorithm. Instead of calling Prefetch|ReadBuffer() for
> each block, ANALYZE now uses the streaming API introduced in b5a9b18cd0.
>
> Author: Nazir Bilal Yavuz
> Reviewed-by: Melanie Plageman
> Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com
> ---
>  src/backend/commands/analyze.c | 89 ++++++++++------------------------
>  1 file changed, 26 insertions(+), 63 deletions(-)

That's a very nice demonstration of how this makes good prefetching easier...




> From 862b7ac81cdafcda7b525e02721da14e46265509 Mon Sep 17 00:00:00 2001
> From: Melanie Plageman <melanieplageman@gmail.com>
> Date: Sun, 7 Apr 2024 15:38:41 -0400
> Subject: [PATCH v10 3/3] Obsolete BlockSampler_HasMore()
> 
> A previous commit stopped using BlockSampler_HasMore() for flow control
> in acquire_sample_rows(). There seems little use now for
> BlockSampler_HasMore(). It should be sufficient to return
> InvalidBlockNumber from BlockSampler_Next() when BlockSample_HasMore()
> would have returned false. Remove BlockSampler_HasMore().
> 
> Author: Melanie Plageman, Nazir Bilal Yavuz
> Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com

The justification here seems somewhat odd. Sure, the previous commit stopped
using BlockSampler_HasMore in acquire_sample_rows - but only because it was
moved to block_sampling_streaming_read_next()?

Greetings,

Andres Freund

On Sun, Apr 07, 2024 at 03:00:00PM -0700, Andres Freund wrote:
> Hi,
> 
> On 2024-04-07 16:59:26 -0400, Melanie Plageman wrote:
> > From 1dc2343661f3edb3b1bc4307afb0e956397eb76c Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Sun, 7 Apr 2024 14:55:22 -0400
> > Subject: [PATCH v10 1/3] Make heapam_scan_analyze_next_[tuple|block] static.
> > 
> > 27bc1772fc81 removed the table AM callbacks scan_analyze_next_block and
> > scan_analzye_next_tuple -- leaving their heap AM implementations only
> > called by acquire_sample_rows().
> 
> Ugh, I don't think 27bc1772fc81 makes much sense. But that's unrelated to this
> thread.  I did raise that separately
> https://www.postgresql.org/message-id/20240407214001.jgpg5q3yv33ve6y3%40awork3.anarazel.de
> 
> Unless I seriously missed something, I see no alternative to reverting that
> commit.

Noted. I'll give up on this refactor then. Lots of churn for no gain.
Attached v11 is just Bilal's v8 patch rebased to apply cleanly and with
a few tweaks (I changed one of the loop conditions. All other changes
are to comments and commit message).

> > @@ -1206,11 +1357,13 @@ acquire_sample_rows(Relation onerel, int elevel,
> >                  break;
> >  
> >              prefetch_block = BlockSampler_Next(&prefetch_bs);
> > -            PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, prefetch_block);
> > +            PrefetchBuffer(scan->rs_base.rs_rd, MAIN_FORKNUM, prefetch_block);
> >          }
> >      }
> >  #endif
> >  
> > +    scan->rs_cbuf = InvalidBuffer;
> > +
> >      /* Outer loop over blocks to sample */
> >      while (BlockSampler_HasMore(&bs))
> >      {
> 
> I don't think it's good to move a lot of code *and* change how it is
> structured in the same commit. Makes it much harder to actually see changes /
> makes git blame harder to use / etc.

Yep.

> > From 90d115c2401567be65bcf64393a6d3b39286779e Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Sun, 7 Apr 2024 15:28:32 -0400
> > Subject: [PATCH v10 2/3] Use streaming read API in ANALYZE
> >
> > The ANALYZE command prefetches and reads sample blocks chosen by a
> > BlockSampler algorithm. Instead of calling Prefetch|ReadBuffer() for
> > each block, ANALYZE now uses the streaming API introduced in b5a9b18cd0.
> >
> > Author: Nazir Bilal Yavuz
> > Reviewed-by: Melanie Plageman
> > Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com
> > ---
> >  src/backend/commands/analyze.c | 89 ++++++++++------------------------
> >  1 file changed, 26 insertions(+), 63 deletions(-)
> 
> That's a very nice demonstration of how this makes good prefetching easier...

Agreed. Yay streaming read API and Bilal!

> > From 862b7ac81cdafcda7b525e02721da14e46265509 Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Sun, 7 Apr 2024 15:38:41 -0400
> > Subject: [PATCH v10 3/3] Obsolete BlockSampler_HasMore()
> > 
> > A previous commit stopped using BlockSampler_HasMore() for flow control
> > in acquire_sample_rows(). There seems little use now for
> > BlockSampler_HasMore(). It should be sufficient to return
> > InvalidBlockNumber from BlockSampler_Next() when BlockSample_HasMore()
> > would have returned false. Remove BlockSampler_HasMore().
> > 
> > Author: Melanie Plageman, Nazir Bilal Yavuz
> > Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com
> 
> The justification here seems somewhat odd. Sure, the previous commit stopped
> using BlockSampler_HasMore in acquire_sample_rows - but only because it was
> moved to block_sampling_streaming_read_next()?

It didn't stop using it. It stopped being useful. The reason it existed,
as far as I can tell, was to use it as the while() loop condition in
acquire_sample_rows(). I think it makes much more sense for
BlockSampler_Next() to return InvalidBlockNumber when there are no more
blocks -- not to assert you don't call it when there aren't any more
blocks.

I didn't want to change BlockSampler_Next() in the same commit as the
streaming read user and we can't remove BlockSampler_HasMore() without
changing BlockSampler_Next().

- Melanie

On Mon, Apr 8, 2024 at 10:26 AM Melanie Plageman
<melanieplageman@gmail.com> wrote:
> On Sun, Apr 07, 2024 at 03:00:00PM -0700, Andres Freund wrote:
> > >  src/backend/commands/analyze.c | 89 ++++++++++------------------------
> > >  1 file changed, 26 insertions(+), 63 deletions(-)
> >
> > That's a very nice demonstration of how this makes good prefetching easier...
>
> Agreed. Yay streaming read API and Bilal!

+1

I found a few comments to tweak, just a couple of places that hadn't
got the memo after we renamed "read stream", and an obsolete mention
of pinning buffers.  I adjusted those directly.

I ran some tests on a random basic Linux/ARM cloud box with a 7.6GB
table, and I got:

                                      cold    hot
master:                             9025ms  199ms
patched, io_combine_limit=1:        9025ms  191ms
patched, io_combine_limit=default:  8729ms  191ms

Despite being random, occasionally some I/Os must get merged, allowing
slightly better random throughput when accessing disk blocks through a
3000 IOPS drinking straw.  Looking at strace, I see 29144 pread* calls
instead of 30071, which fits that theory.  Let's see... if you roll a
fair 973452-sided dice 30071 times, how many times do you expect to
roll consecutive numbers?  Each time you roll there is a 1/973452
chance that you get the last number + 1, and we have 30071 tries
giving 30071/973452 = ~3%.  9025ms minus 3% is 8754ms.  Seems about
right.

I am not sure why the hot number is faster exactly.  (Anecdotally, I
did notice that in the cases that beat master semi-unexpectedly like
this, my software memory prefetch patch doesn't help or hurt, while in
some cases and on some CPUs there is little difference, and then that
patch seems to get a speed-up like this, which might be a clue.
*Shrug*, investigation needed.)

Pushed.  Thanks Bilal and reviewers!

On Mon, Apr 8, 2024 at 10:26 AM Melanie Plageman
<melanieplageman@gmail.com> wrote:
> On Sun, Apr 07, 2024 at 03:00:00PM -0700, Andres Freund wrote:
> > On 2024-04-07 16:59:26 -0400, Melanie Plageman wrote:
> > > From 862b7ac81cdafcda7b525e02721da14e46265509 Mon Sep 17 00:00:00 2001
> > > From: Melanie Plageman <melanieplageman@gmail.com>
> > > Date: Sun, 7 Apr 2024 15:38:41 -0400
> > > Subject: [PATCH v10 3/3] Obsolete BlockSampler_HasMore()
> > >
> > > A previous commit stopped using BlockSampler_HasMore() for flow control
> > > in acquire_sample_rows(). There seems little use now for
> > > BlockSampler_HasMore(). It should be sufficient to return
> > > InvalidBlockNumber from BlockSampler_Next() when BlockSample_HasMore()
> > > would have returned false. Remove BlockSampler_HasMore().
> > >
> > > Author: Melanie Plageman, Nazir Bilal Yavuz
> > > Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com
> >
> > The justification here seems somewhat odd. Sure, the previous commit stopped
> > using BlockSampler_HasMore in acquire_sample_rows - but only because it was
> > moved to block_sampling_streaming_read_next()?
>
> It didn't stop using it. It stopped being useful. The reason it existed,
> as far as I can tell, was to use it as the while() loop condition in
> acquire_sample_rows(). I think it makes much more sense for
> BlockSampler_Next() to return InvalidBlockNumber when there are no more
> blocks -- not to assert you don't call it when there aren't any more
> blocks.
>
> I didn't want to change BlockSampler_Next() in the same commit as the
> streaming read user and we can't remove BlockSampler_HasMore() without
> changing BlockSampler_Next().

I agree that the code looks useless if one condition implies the
other, but isn't it good to keep that cross-check, perhaps
reformulated as an assertion?  I didn't look too hard at the maths, I
just saw the words "It is not obvious that this code matches Knuth's
Algorithm S ..." and realised I'm not sure I have time to develop a
good opinion about this today.  So I'll leave the 0002 change out for
now, as it's a tidy-up that can easily be applied in the next cycle.

Hi,

On Wed, 3 Apr 2024 at 22:25, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
>
> Hi,
>
> Thank you for looking into this!
>
> On Wed, 3 Apr 2024 at 20:17, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> >
> > On 03/04/2024 13:31, Nazir Bilal Yavuz wrote:
> > > Streaming API has been committed but the committed version has a minor
> > > change, the read_stream_begin_relation function takes Relation instead
> > > of BufferManagerRelation now. So, here is a v5 which addresses this
> > > change.
> >
> > I'm getting a repeatable segfault / assertion failure with this:
> >
> > postgres=# CREATE TABLE tengiga (i int, filler text) with (fillfactor=10);
> > CREATE TABLE
> > postgres=# insert into tengiga select g, repeat('x', 900) from
> > generate_series(1, 1400000) g;
> > INSERT 0 1400000
> > postgres=# set default_statistics_target = 10; ANALYZE tengiga;
> > SET
> > ANALYZE
> > postgres=# set default_statistics_target = 100; ANALYZE tengiga;
> > SET
> > ANALYZE
> > postgres=# set default_statistics_target =1000; ANALYZE tengiga;
> > SET
> > server closed the connection unexpectedly
> >         This probably means the server terminated abnormally
> >         before or while processing the request.
> >
> > TRAP: failed Assert("BufferIsValid(hscan->rs_cbuf)"), File:
> > "heapam_handler.c", Line: 1079, PID: 262232
> > postgres: heikki postgres [local]
> > ANALYZE(ExceptionalCondition+0xa8)[0x56488a0de9d8]
> > postgres: heikki postgres [local]
> > ANALYZE(heapam_scan_analyze_next_block+0x63)[0x5648899ece34]
> > postgres: heikki postgres [local] ANALYZE(+0x2d3f34)[0x564889b6af34]
> > postgres: heikki postgres [local] ANALYZE(+0x2d2a3a)[0x564889b69a3a]
> > postgres: heikki postgres [local] ANALYZE(analyze_rel+0x33e)[0x564889b68fa9]
> > postgres: heikki postgres [local] ANALYZE(vacuum+0x4b3)[0x564889c2dcc0]
> > postgres: heikki postgres [local] ANALYZE(ExecVacuum+0xd6f)[0x564889c2d7fe]
> > postgres: heikki postgres [local]
> > ANALYZE(standard_ProcessUtility+0x901)[0x564889f0b8b9]
> > postgres: heikki postgres [local]
> > ANALYZE(ProcessUtility+0x136)[0x564889f0afb1]
> > postgres: heikki postgres [local] ANALYZE(+0x6728c8)[0x564889f098c8]
> > postgres: heikki postgres [local] ANALYZE(+0x672b3b)[0x564889f09b3b]
> > postgres: heikki postgres [local] ANALYZE(PortalRun+0x320)[0x564889f09015]
> > postgres: heikki postgres [local] ANALYZE(+0x66b2c6)[0x564889f022c6]
> > postgres: heikki postgres [local]
> > ANALYZE(PostgresMain+0x80c)[0x564889f06fd7]
> > postgres: heikki postgres [local] ANALYZE(+0x667876)[0x564889efe876]
> > postgres: heikki postgres [local]
> > ANALYZE(postmaster_child_launch+0xe6)[0x564889e1f4b3]
> > postgres: heikki postgres [local] ANALYZE(+0x58e68e)[0x564889e2568e]
> > postgres: heikki postgres [local] ANALYZE(+0x58b7f0)[0x564889e227f0]
> > postgres: heikki postgres [local]
> > ANALYZE(PostmasterMain+0x152b)[0x564889e2214d]
> > postgres: heikki postgres [local] ANALYZE(+0x4444b4)[0x564889cdb4b4]
> > /lib/x86_64-linux-gnu/libc.so.6(+0x2724a)[0x7f7d83b6724a]
> > /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7f7d83b67305]
> > postgres: heikki postgres [local] ANALYZE(_start+0x21)[0x564889971a61]
> > 2024-04-03 20:15:49.157 EEST [262101] LOG:  server process (PID 262232)
> > was terminated by signal 6: Aborted
>
> I realized the same error while working on Jakub's benchmarking results.
>
> Cause: I was using the nblocks variable to check how many blocks will
> be returned from the streaming API. But I realized that sometimes the
> number returned from BlockSampler_Init() is not equal to the number of
> blocks that BlockSampler_Next() will return as BlockSampling algorithm
> decides how many blocks to return on the fly by using some random
> seeds.

I wanted to re-check this problem and I realized that I was wrong. I
tried using nblocks again and this time there was no failure. I looked
at block sampling logic and I am pretty sure that BlockSampler_Init()
function correctly returns the number of blocks that
BlockSampler_Next() will return. It seems 158f581923 fixed this issue
as well.

-- 
Regards,
Nazir Bilal Yavuz
Microsoft

Hi,

On Mon, 8 Apr 2024 at 04:21, Thomas Munro <thomas.munro@gmail.com> wrote:
>
> Pushed.  Thanks Bilal and reviewers!

I wanted to discuss what will happen to this patch now that
27bc1772fc8 is reverted. I am continuing this thread but I can create
another thread if you prefer so.

After the revert of 27bc1772fc8, acquire_sample_rows() became table-AM
agnostic again. So, read stream changes might have to be pushed down
now but there are a couple of roadblocks like Melanie mentioned [1]
before.

Quote from Melanie [1]:

On Thu, 11 Apr 2024 at 19:19, Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> I am working on pushing streaming ANALYZE into heap AM code, and I ran
> into a few roadblocks.
>
> If we want ANALYZE to make the ReadStream object in heap_beginscan()
> (like the read stream implementation of heap sequential and TID range
> scans do), I don't see any way around changing the scan_begin table AM
> callback to take a BufferAccessStrategy at the least (and perhaps also
> the BlockSamplerData).
>
> read_stream_begin_relation() doesn't just save the
> BufferAccessStrategy in the ReadStream, it uses it to set various
> other things in the ReadStream object. callback_private_data (which in
> ANALYZE's case is the BlockSamplerData) is simply saved in the
> ReadStream, so it could be set later, but that doesn't sound very
> clean to me.
>
> As such, it seems like a cleaner alternative would be to add a table
> AM callback for creating a read stream object that takes the
> parameters of read_stream_begin_relation(). But, perhaps it is a bit
> late for such additions.

If we do not want to add a new table AM callback like Melanie
mentioned, it is pretty much required to pass BufferAccessStrategy and
BlockSamplerData to the initscan().

> It also opens us up to the question of whether or not sequential scan
> should use such a callback instead of making the read stream object in
> heap_beginscan().
>
> I am happy to write a patch that does any of the above. But, I want to
> raise these questions, because perhaps I am simply missing an obvious
> alternative solution.

I wonder the same, I could not think of any alternative solution to
this problem.

Another quote from Melanie [2] in the same thread:

On Thu, 11 Apr 2024 at 20:48, Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> I will also say that, had this been 6 months ago, I would probably
> suggest we restructure ANALYZE's table AM interface to accommodate
> read stream setup and to address a few other things I find odd about
> the current code. For example, I think creating a scan descriptor for
> the analyze scan in acquire_sample_rows() is quite odd. It seems like
> it would be better done in the relation_analyze callback. The
> relation_analyze callback saves some state like the callbacks for
> acquire_sample_rows() and the Buffer Access Strategy. But at least in
> the heap implementation, it just saves them in static variables in
> analyze.c. It seems like it would be better to save them in a useful
> data structure that could be accessed later. We have access to pretty
> much everything we need at that point (in the relation_analyze
> callback). I also think heap's implementation of
> table_beginscan_analyze() doesn't need most of
> heap_beginscan()/initscan(), so doing this instead of something
> ANALYZE specific seems more confusing than helpful.

If we want to implement ANALYZE specific counterparts of
heap_beginscan()/initscan(); we may think of passing
BufferAccessStrategy and BlockSamplerData to them.

Also, there is an ongoing(?) discussion about a few problems /
improvements about the acquire_sample_rows() mentioned at the end of
the 'Table AM Interface Enhancements' thread [3]. Should we wait for
these discussions to be resolved or can we resume working on this
patch?

Any kind of feedback would be appreciated.

[1] https://www.postgresql.org/message-id/CAAKRu_ZxU6hucckrT1SOJxKfyN7q-K4KU1y62GhDwLBZWG%2BROg%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAAKRu_YkphAPNbBR2jcLqnxGhDEWTKhYfLFY%3D0R_oG5LHBH7Gw%40mail.gmail.com
[3] https://www.postgresql.org/message-id/flat/CAPpHfdurb9ycV8udYqM%3Do0sPS66PJ4RCBM1g-bBpvzUfogY0EA%40mail.gmail.com

-- 
Regards,
Nazir Bilal Yavuz
Microsoft

Hi,

On Mon, 29 Apr 2024 at 18:41, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
>
> Hi,
>
> On Mon, 8 Apr 2024 at 04:21, Thomas Munro <thomas.munro@gmail.com> wrote:
> >
> > Pushed.  Thanks Bilal and reviewers!
>
> I wanted to discuss what will happen to this patch now that
> 27bc1772fc8 is reverted. I am continuing this thread but I can create
> another thread if you prefer so.

041b96802ef is discussed in the 'Table AM Interface Enhancements'
thread [1]. The main problems discussed about this commit is that the
read stream API is not pushed to the heap-specific code and, because
of that, the other AM implementations need to use read streams. To
push read stream API to the heap-specific code, it is pretty much
required to pass BufferAccessStrategy and BlockSamplerData to the
initscan().

I am sharing the alternative version of this patch. The first patch
just reverts 041b96802ef and the second patch is the alternative
version.

In this alternative version, the read stream API is not pushed to the
heap-specific code, but it is controlled by the heap-specific code.
The SO_USE_READ_STREAMS_IN_ANALYZE flag is introduced and set in the
heap-specific code if the scan type is 'ANALYZE'. This flag is used to
decide whether streaming API in ANALYZE will be used or not. If this
flag is set, this means heap AMs and read stream API will be used. If
it is not set, this means heap AMs will not be used and code falls
back to the version before read streams.

Pros of the alternative version:

* The existing AM implementations other than heap AM can continue to
use their AMs without any change.
* AM implementations other than heap do not need to use read streams.
* Upstream code uses the read stream API and benefits from that.

Cons of the alternative version:

* 6 if cases are added to the acquire_sample_rows() function and 3 of
them are in the while loop.
* Because of these changes, the code looks messy.

Any kind of feedback would be appreciated.

[1] https://www.postgresql.org/message-id/flat/CAPpHfdurb9ycV8udYqM%3Do0sPS66PJ4RCBM1g-bBpvzUfogY0EA%40mail.gmail.com

-- 
Regards,
Nazir Bilal Yavuz
Microsoft

On Wed, May 15, 2024 at 2:18 PM Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
>
> On Mon, 29 Apr 2024 at 18:41, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
> >
> > On Mon, 8 Apr 2024 at 04:21, Thomas Munro <thomas.munro@gmail.com> wrote:
> > I wanted to discuss what will happen to this patch now that
> > 27bc1772fc8 is reverted. I am continuing this thread but I can create
> > another thread if you prefer so.
>
> 041b96802ef is discussed in the 'Table AM Interface Enhancements'
> thread [1]. The main problems discussed about this commit is that the
> read stream API is not pushed to the heap-specific code and, because
> of that, the other AM implementations need to use read streams. To
> push read stream API to the heap-specific code, it is pretty much
> required to pass BufferAccessStrategy and BlockSamplerData to the
> initscan().
>
> I am sharing the alternative version of this patch. The first patch
> just reverts 041b96802ef and the second patch is the alternative
> version.
>
> In this alternative version, the read stream API is not pushed to the
> heap-specific code, but it is controlled by the heap-specific code.
> The SO_USE_READ_STREAMS_IN_ANALYZE flag is introduced and set in the
> heap-specific code if the scan type is 'ANALYZE'. This flag is used to
> decide whether streaming API in ANALYZE will be used or not. If this
> flag is set, this means heap AMs and read stream API will be used. If
> it is not set, this means heap AMs will not be used and code falls
> back to the version before read streams.

Personally, I think the alternative version here is the best option
other than leaving what is in master. However, I would vote for
keeping what is in master because 1) where we are in the release
timeline and 2) the acquire_sample_rows() code, before streaming read,
was totally block-based anyway.

If we kept what was in master, do we need to document for table AMs
how to use read_stream_next_buffer() or can we assume they will look
at the heap AM implementation?

- Melanie

On Thu, Aug 22, 2024 at 7:31 PM Mats Kindahl <mats@timescale.com> wrote:
> The alternate version proposed by Nazir allows you to decide which interface to use.
> Reverting the patch entirely would also solve the problem.
> Passing down the block sampler and the strategy to scan_begin() and move the ReadStream setup in analyze.c into
initscan()in heapam.c, but this requires adding new parameters to this function. 
> Having accessors that allow you to get the block sampler and strategy from the ReadStream object.

I'm a bit confused about how it can make sense to use the same
BlockSampler with a side relation/fork.  Could you point me at the
code?

> It would be great if this could be fixed before the PG17 release now that 27bc1772fc8 was reverted.

Ack.  Thinking...

Random thought: is there a wiki page or something where we can find
out about all the table AM projects?  For the successor to
27bc1772fc8, I hope they'll be following along.

Thanks for the explanation.  I think we should revert it.  IMHO it was
a nice clean example of a streaming transformation, but unfortunately
it transformed an API that nobody liked in the first place, and broke
some weird and wonderful workarounds.  Let's try again in 18.

On Wed, Sep 4, 2024 at 6:38 AM Thomas Munro <thomas.munro@gmail.com> wrote:
> Thanks for the explanation.  I think we should revert it.  IMHO it was
> a nice clean example of a streaming transformation, but unfortunately
> it transformed an API that nobody liked in the first place, and broke
> some weird and wonderful workarounds.  Let's try again in 18.

The problem I have with this is that we just released RC1. I suppose
if we have to make this change it's better to do it sooner than later,
but are we sure we want to whack this around this close to final
release?

--
Robert Haas
EDB: http://www.enterprisedb.com

On Thu, Sep 5, 2024 at 3:36 AM Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Sep 4, 2024 at 6:38 AM Thomas Munro <thomas.munro@gmail.com> wrote:
> > Thanks for the explanation.  I think we should revert it.  IMHO it was
> > a nice clean example of a streaming transformation, but unfortunately
> > it transformed an API that nobody liked in the first place, and broke
> > some weird and wonderful workarounds.  Let's try again in 18.
>
> The problem I have with this is that we just released RC1. I suppose
> if we have to make this change it's better to do it sooner than later,
> but are we sure we want to whack this around this close to final
> release?

I hear you.  But I definitely don't want to (and likely can't at this
point) make any of the other proposed changes, and I also don't want
to break Timescale.  That seems to leave only one option: go back to
the v16 API for RC2, and hope that the ongoing table AM discussions
for v18 (CF #4866) will fix all the problems for the people whose TAMs
don't quack like a "heap", and the people whose TAMs do and who would
not like to duplicate the code, and the people who want streaming I/O.

On Thu, Sep 5, 2024 at 6:45 PM Mats Kindahl <mats@timescale.com> wrote:
> Forgive me for asking, but I am not entirely sure why the ReadStream struct is opaque. The usual reasons are:
>
> You want to provide an ABI to allow extensions to work with new major versions without re-compiling. Right now it is
necessaryto recompile extensions anyway, this does not seem to apply. (Because there are a lot of other changes that
youneed when switching versions because of the lack of a stable ABI for other parts of the code. However, it might be
thatthe goal is to support it eventually, and then it would make sense to start making structs opaque.) 
> You want to ensure that you can make modifications inside a major version without breaking ABIs and requiring a
re-compile.In this case, you could still follow safe practice of adding new fields last, not relying on the size of the
structfor anything (e.g., no arrays of these structures, just pointers to them), etc. However, if you want to be very
safeand support very drastic changes inside a major version, it needs to be opaque, so this could be the reason. 
>
> Is it either of these reasons, or is there another reason?
>
> Making the ReadStream API non-opaque (that is, moving the definition to the header file) would at least solve our
problem(unless I am mistaken). However, I am ignorant about long-term plans which might affect this, so there might be
agood reason to revert it for reasons I am not aware of. 

The second thing.  Also there are very active plans[1] to change the
internal design of ReadStream in 18, since the goal is to drive true
asynchronous I/O, and the idea of ReadStream was to create a simple
API to let many consumers start using it, so that we can drive
efficient modern system interfaces below that API, so having people
depending on how it works would not be great.

But let's talk about how that would actually look, for example if we
exposed the struct or you took a photocopy of it...  I think your idea
must be something like: if you could access struct ReadStream's
internals, you could replace stream->callback with an interceptor
callback, and if the BlockSampler had been given the fake N + M
relation size, the interceptor could overwrite
stream->ios[next_io_index].op.smgr and return x - N if the intercepted
callback returned x >= N.  (Small detail: need to check
stream->fast_path and use 0 instead or something like that, but maybe
we could change that.)  One minor problem that jumps out is that
read_stream.c could inappropriately merge blocks from the two
relations into one I/O.  Hmm, I guess you'd have to teach the
interceptor not to allow that: if switching between the two relation,
and if the block number would coincide with
stream->pending_read_blocknum + stream->pending_read_nblocks, it would
need to pick a new block instead (interfering with the block sampling
algorithm, but only very rarely).  Is this what you had in mind, or
something else?

(BTW I have a patch to teach read_stream.c about multi-smgr-relation
streams, by adding a different constructor with a different callback
that returns smgr, fork, block instead of just the block, but it
didn't make it into 17.)

[1] https://www.postgresql.org/message-id/flat/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah@brqs62irg4dt

Hi,

On Thu, Sep 05, 2024 at 09:12:07PM +1200, Thomas Munro wrote:
> On Thu, Sep 5, 2024 at 6:45 PM Mats Kindahl <mats@timescale.com> wrote:
> > Making the ReadStream API non-opaque (that is, moving the definition
> > to the header file) would at least solve our problem (unless I am
> > mistaken). However, I am ignorant about long-term plans which might
> > affect this, so there might be a good reason to revert it for
> > reasons I am not aware of.
> 
> The second thing.

I am a bit confused about the status of this thread. Robert mentioned
RC1, so I guess it pertains to v17 but I don't see it on the open item
wiki list?

Does the above mean you are going to revert it for v17, Thomas? And if
so, what exactly? The ANALYZE changes on top of the streaming read API
or something else about that API that is being discussed on this thread?

I am also asking because this feature (i.e. Use streaming read API in
ANALYZE) is being mentioned in the release announcement and that was
just frozen for translations.


Michael

On Tue, Sep 10, 2024 at 10:27 AM Thomas Munro <thomas.munro@gmail.com> wrote:
> Mats, what do you think about
> this?  (I haven't tried to preserve the prefetching behaviour, which
> probably didn't actually too work for you in v16 anyway at a guess,
> I'm just looking for the absolute simplest thing we can do to resolve
> this API mismatch.)  TimeScale could then continue to use its v16
> coding to handle the two-relations-in-a-trenchcoat problem, and we
> could continue discussing how to make v18 better.

. o O { Spitballing here: if we add that tiny function I showed to get
you unstuck for v17, then later in v18, if we add a multi-relation
ReadStream constructor/callback (I have a patch somewhere, I want to
propose that as it is needed for streaming recovery), you could
construct a new ReadSteam of your own that is daisy-chained from that
one.  You could keep using your N + M block numbering scheme if you
want to, and the callback of the new stream could decode the block
numbers and redirect to the appropriate relation + real block number.
That way you'd get I/O concurrency for both relations (for now just
read-ahead advice, but see Andres's AIO v2 thread).  That'd
essentially be a more supported version of the 'access the struct
internals' idea (or at least my understanding of what you had in
mind), through daisy-chained streams.  A little weird maybe, and maybe
the redesign work will result in something completely
different/better... just a thought... }

On Sun, Sep 15, 2024 at 12:14 AM Mats Kindahl <mats@timescale.com> wrote:
> I used the combination of your patch and making the computation of vacattrstats for a relation available through the
APIand managed to implement something that I think does the right thing. (I just sampled a few different statistics to
checkif they seem reasonable, like most common vals and most common freqs.) See attached patch. 

Cool.  I went ahead and committed that small new function and will
mark the open item closed.

> I need the vacattrstats to set up the two streams for the internal relations. I can just re-implement them in the
sameway as is already done, but this seems like a small change that avoids unnecessary code duplication. 

Unfortunately we're not in a phase where we can make non-essential
changes, we're right about to release and we're only committing fixes,
and it seems like you have a way forward (albeit with some
duplication).  We can keep talking about that for v18.

From your earlier email:
> I'll take a look at the thread. I really think the ReadStream abstraction is a good step in the right direction.

Here's something you or your colleagues might be interested in: I was
looking around for a fun extension to streamify as a demo of the
technology, and I finished up writing a quick patch to streamify
pgvector's HNSW index scan, which worked well enough to share[1] (I
think it should in principle be able to scale with the number of graph
connections, at least 16x), but then people told me that it's of
limited interest because everybody knows that HNSW indexes have to fit
in memory (I think there may also be memory prefetch streaming
opportunities, unexamined for now).  But that made me wonder what the
people with the REALLY big indexes do for hyperdimensional graph
search on a scale required to build Skynet, and that led me back to
Timescale pgvectorscale[2].  I see two obvious signs that this thing
is eminently and profitably streamifiable: (1) The stated aim is
optimising for indexes that don't fit in memory, hence "Disk" in the
name of the research project it is inspired by, (2) I see that
DIskANN[3] is aggressively using libaio (Linux) and overlapped/IOCP
(Windows).  So now I am waiting patiently for a Rustacean to show up
with patches for pgvectorscale to use ReadStream, which would already
get read-ahead advice and vectored I/O (Linux, macOS, FreeBSD soon
hopefully), and hopefully also provide a nice test case for the AIO
patch set which redirects buffer reads through io_uring (Linux,
basically the newer better libaio) or background I/O workers (other
OSes, which works surprisingly competitively).  Just BTW for
comparison with DiskANN we have also had early POC-quality patches
that drive AIO with overlapped/IOCP (Windows) which will eventually be
rebased and proposed (Windows isn't really a primary target but we
wanted to validate that the stuff we're working on has abstractions
that will map to the obvious system APIs found in the systems
PostgreSQL targets).  For completeness, I've also had it mostly
working on the POSIX AIO of FreeBSD, HP-UX and AIX (though we dropped
support for those last two so that was a bit of a dead end).

[1] https://www.postgresql.org/message-id/flat/CA%2BhUKGJ_7NKd46nx1wbyXWriuZSNzsTfm%2BrhEuvU6nxZi3-KVw%40mail.gmail.com
[2] https://github.com/timescale/pgvectorscale
[3] https://github.com/microsoft/DiskANN