Thread: EXPLAIN ANALYZE printing logical and hardware I/O per-node

I've wanted for a long time to have EXPLAIN ANALYZE output per-node I/O usage.
This would be especially useful if we could distinguish hardware versus
logical I/O though. And I always thought that would be very hard.

My thought in the past was that would could do it on Solaris by having
Postgres use DTrace directly via its (undocumented but existing) programmatic
interface.

For other operating systems it was tempting to suggest just timing the read(2)
call to see if it took too long to be a logical operation. The problem there
is that gettimeofday would impose far too much overhead to make that practical
(or even be precise enough to work properly).

But it occurred to me just now that the hardware instruction counter available
on just about every platform would be good enough for a heuristic guess at
whether the read(2) was cached. The problem generally with using the hardware
instruction counter is that it's not necessarily in sync between processors
and might therefore run backwards or skip time forwards. This is a problem for
profiling but if all we care about is a boolean guess at whether the request
was satisfied quickly from cache then any such skipping forward or backward
would represent a context switch which we could just toss in the hardware i/o
bucket. It doesn't matter exactly how long the hardware i/o took, only that
there was one.

To that end I would love to see something like:
                                                QUERY PLAN                                                  
-------------------------------------------------------------------------------------------------------------Bitmap
HeapScan on h  (cost=8.52..16.45 rows=2 width=512) (actual time=78.926..87.708 rows=2 loops=1 logical-I/O=2
physical-I/O=1) Recheck Cond: (i = ANY ('{100,1000}'::integer[]))  ->  Bitmap Index Scan on hi  (cost=0.00..8.52 rows=2
width=0)(actual time=74.539..74.539 rows=2 loops=1 logical-I/O=2 physical-I/O=2))        Index Cond: (i = ANY
('{100,1000}'::integer[]))Totalruntime: 87.820 ms
 


--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com Ask me about EnterpriseDB's PostGIS support!

On Fri, 2007-12-14 at 15:47 +0000, Gregory Stark wrote:
> I've wanted for a long time to have EXPLAIN ANALYZE output per-node I/O usage.
> This would be especially useful if we could distinguish hardware versus
> logical I/O though. And I always thought that would be very hard.
> 
> My thought in the past was that would could do it on Solaris by having
> Postgres use DTrace directly via its (undocumented but existing) programmatic
> interface.
> 
> For other operating systems it was tempting to suggest just timing the read(2)
> call to see if it took too long to be a logical operation. The problem there
> is that gettimeofday would impose far too much overhead to make that practical
> (or even be precise enough to work properly).
> 
> But it occurred to me just now that the hardware instruction counter available
> on just about every platform would be good enough for a heuristic guess at
> whether the read(2) was cached.

I'm skeptical that this would be reliable enough to be very useful,
especially in the face of concurrent, unpredictable system activity on a
busy system. I agree that it would be useful information, though.
Perhaps a useful first step would be to teach EXPLAIN ANALYZE to report
the number of logical and physical I/Os from Postgres' perspective (i.e.
physical I/O just means we need to go to the kernel).

>  The problem generally with using the hardware
> instruction counter is that it's not necessarily in sync between processors
> and might therefore run backwards or skip time forwards. This is a problem for
> profiling but if all we care about is a boolean guess at whether the request
> was satisfied quickly from cache then any such skipping forward or backward
> would represent a context switch which we could just toss in the hardware i/o
> bucket. It doesn't matter exactly how long the hardware i/o took, only that
> there was one.
> 
> To that end I would love to see something like:
> 
>                                                  QUERY PLAN                                                  
> -------------------------------------------------------------------------------------------------------------
>  Bitmap Heap Scan on h  (cost=8.52..16.45 rows=2 width=512) (actual time=78.926..87.708 rows=2 loops=1 logical-I/O=2
physical-I/O=1)
>    Recheck Cond: (i = ANY ('{100,1000}'::integer[]))
>    ->  Bitmap Index Scan on hi  (cost=0.00..8.52 rows=2 width=0) (actual time=74.539..74.539 rows=2 loops=1
logical-I/O=2physical-I/O=2))
 
>          Index Cond: (i = ANY ('{100,1000}'::integer[]))
>  Total runtime: 87.820 ms
> 
> 

Hi,<br />    I already made a discussion about it. We can view the Logical I/Os. If we enable the log_statement_stats
inthe conf file and apply the following patch, it is possible. But putting it in Explain analyze makes more sense to
me.<br /><br /><br /><br />*** postgresql-8.3beta1/src/backend/storage/buffer<div class="ArwC7c ckChnd"
id="1fcg">/bufmgr.c    Tue Sep 25<br />18:11:48 2007<br />--- postgresql-8.3patch/src/backend/storage/buffer/bufmgr.c  
 Fri Oct 19 <br />23:18:36 2007<br />***************<br />*** 1470,1477 ****<br />                localhitrate =
(float)LocalBufferHitCount *100.0 / ReadLocalBufferCount;<br /><br />        appendStringInfo(&str,<br />!      
"!\tSharedblocks: %10ld read, %10ld written, buffer hit rate = %.2f%%\n", <br />!                              
ReadBufferCount- BufferHitCount, BufferFlushCount, hitrate);<br />        appendStringInfo(&str,<br />      
 "!\tLocal blocks: %10ld read, %10ld written, buffer hit rate = %.2f%%\n", <br />                                      
 ReadLocalBufferCount - LocalBufferHitCount,<br />LocalBufferFlushCount, localhitrate);<br />--- 1470,1477 ----<br />  
            localhitrate = (float) LocalBufferHitCount *100.0 / ReadLocalBufferCount; <br /><br />      
 appendStringInfo(&str,<br/>!       "!\tShared blocks: %10ld Logical Reads, %10ld Physical Reads, %10ld<br
/>written,buffer hit rate = %.2f%%\n",<br />!                               ReadBufferCount, ReadBufferCount -
BufferHitCount,<br />BufferFlushCount, hitrate);<br />        appendStringInfo(&str,<br />        "!\tLocal
 blocks:%10ld read, %10ld written, buffer hit rate = %.2f%%\n",<br />                                        
ReadLocalBufferCount- LocalBufferHitCount, <br />LocalBufferFlushCount, localhitrate);<br /><br /><br /><br /></div><br
clear="all"/><br />-- <br />Thanks,<br />Gokul.<br />CertoSQL Project,<br />Allied Solution Group.<br />(<a
href="http://www.alliedgroups.com">www.alliedgroups.com</a>)  

"Gokulakannan Somasundaram" <gokul007@gmail.com> writes:

> Hi,
>     I already made a discussion about it. We can view the Logical I/Os. If
> we enable the log_statement_stats in the conf file and apply the following
> patch, it is possible. But putting it in Explain analyze makes more sense to
> me.

I was going to say that I'm really only interested in physical I/O. Logical
I/O which is satisfied by the kernel cache is only marginally interesting and
buffer fetches from Postgres's shared buffer is entirely uninteresting from
the point of view of trying to figure out what is slowing down a query.

However I suppose that's not true. There are other reasons why buffer fetches
could be interesting. In particular I imagine when users post explain analyzes
it would give us a good idea of whether their tables or bloated or their
tuples are extremely wide (in cases where the planner gets it wrong).

But I do think that showing logical I/Os without even an heuristic based
measurement of actual physical i/o is pretty useless. It will make people
think they want to grow their shared buffers to cover all of memory.

--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com Ask me about EnterpriseDB's PostGIS support!

Gokulakannan Somasundaram wrote:
> I was going to say that I'm really only interested in physical I/O. Logical
>> I/O which is satisfied by the kernel cache is only marginally interesting
>> and
>> buffer fetches from Postgres's shared buffer is entirely uninteresting
>> from
>> the point of view of trying to figure out what is slowing down a query.
> 
> Ok the Physical I/Os are already visible, if you enable log_statement_stats.

I think you missed the point. What log_statement_stats shows are not 
physical I/Os, they're read() system calls. Unfortunately there's no 
direct way to tell if a read() is satisfied from OS cache or not. Greg's 
suggestion was about how to do that.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com

<br /><br /><div class="gmail_quote">On Dec 16, 2007 1:03 AM, Heikki Linnakangas <<a
href="mailto:heikki@enterprisedb.com">heikki@enterprisedb.com</a>>wrote:<br /><blockquote class="gmail_quote"
style="border-left:1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div
class="Ih2E3d">GokulakannanSomasundaram wrote:<br />> I was going to say that I'm really only interested in physical
I/O.Logical<br />>> I/O which is satisfied by the kernel cache is only marginally interesting <br />>>
and<br/>>> buffer fetches from Postgres's shared buffer is entirely uninteresting<br />>> from<br
/>>>the point of view of trying to figure out what is slowing down a query.<br />><br />> Ok the Physical
I/Osare already visible, if you enable log_statement_stats. <br /><br /></div>I think you missed the point. What
log_statement_statsshows are not<br />physical I/Os, they're read() system calls. Unfortunately there's no<br />direct
wayto tell if a read() is satisfied from OS cache or not. Greg's <br />suggestion was about how to do that.<br
/></blockquote></div><br/>Oh OK. Thanks for clarifying..<br clear="all" /><br />-- <br />Thanks,<br />Gokul.<br
/>CertoSQLProject,<br />Allied Solution Group.<br />(<a href="http://www.alliedgroups.com"> www.alliedgroups.com</a>)  

On Dec 14, 2007, at 11:10 PM, Neil Conway wrote:
>> But it occurred to me just now that the hardware instruction  
>> counter available
>> on just about every platform would be good enough for a heuristic  
>> guess at
>> whether the read(2) was cached.
>
> I'm skeptical that this would be reliable enough to be very useful,
> especially in the face of concurrent, unpredictable system activity  
> on a
> busy system. I agree that it would be useful information, though.
> Perhaps a useful first step would be to teach EXPLAIN ANALYZE to  
> report
> the number of logical and physical I/Os from Postgres' perspective  
> (i.e.
> physical I/O just means we need to go to the kernel).


*watches hands wave*

If we assume that what we *really* want to see the difference of is a  
cache IO vs one that truly hits a platter, I don't see how you could  
come up with enough variation to account for that. Unless you're  
super lucky and the sector you need is just about to hit the head  
when you issue the request, you're going to have a delay measured in  
milliseconds. 1ms on a modern CPU is over 100,000 cycles. That's a  
lot of instructions...

Someone want to throw together some code that actually measures this?  
Maybe something that keeps a histogram of how many instructions take  
place per I/O request? If it turns out that counters do vary too much  
between CPUs, there might be ways that we can account for that.

Also, has anyone looked into adding a class of system calls that  
would actually tell us if the kernel issued physical IO? I find it  
hard to believe that other RDBMSes wouldn't like to have that info...
-- 
Decibel!, aka Jim C. Nasby, Database Architect  decibel@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828

On Mon, 17 Dec 2007, Decibel! wrote:

> Someone want to throw together some code that actually measures this? Maybe 
> something that keeps a histogram of how many instructions take place per I/O 
> request? If it turns out that counters do vary too much between CPUs, there 
> might be ways that we can account for that.

I'd expect the easiest way to do a proof of concept here would be to use 
the Linux oprofile tool.  That's already abstracted away the differences 
in hardware counters and provides a relatively simple interface to collect 
the data without getting bogged down with implementation there.  Starting 
from scratch, going right to the hardware counters and building from 
there, is a big project--they've been hacking on oprofile for almost six 
years now and still aren't suggesting it's release quality yet.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD

On 12/17/07, Decibel! <decibel@decibel.org> wrote:

> Also, has anyone looked into adding a class of system calls that
> would actually tell us if the kernel issued physical IO? I find it
> hard to believe that other RDBMSes wouldn't like to have that info...

Non-blocking style interfaces can help here. On Windows, for instance,
a read returns data at the call site if it was satisfied by cache,
instead of invoking the asynchronous notification.

"Decibel!" <decibel@decibel.org> writes:

> Also, has anyone looked into adding a class of system calls that  would
> actually tell us if the kernel issued physical IO? I find it  hard to believe
> that other RDBMSes wouldn't like to have that info...

Yeah, I think that's called "DTrace"

--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com Ask me about EnterpriseDB's 24x7 Postgres support!

On Dec 18, 2007, at 3:32 AM, Gregory Stark wrote:
>> Also, has anyone looked into adding a class of system calls that   
>> would
>> actually tell us if the kernel issued physical IO? I find it  hard  
>> to believe
>> that other RDBMSes wouldn't like to have that info...
>
> Yeah, I think that's called "DTrace"


That seems like way overkill...

When a read() call returns, surely the kernel knows whether it  
actually issued a physical read request to satisfy that. I don't see  
any reason why you couldn't have a version of read() that returns  
that information. I also rather doubt that we're the only userland  
software that would make use of that.
-- 
Decibel!, aka Jim C. Nasby, Database Architect  decibel@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828

"Decibel!" <decibel@decibel.org> writes:

> When a read() call returns, surely the kernel knows whether it  actually issued
> a physical read request to satisfy that. I don't see  any reason why you
> couldn't have a version of read() that returns  that information. I also rather
> doubt that we're the only userland  software that would make use of that.

I'm told this exists on Windows for the async interface. But AFAIK it doesn't
on Unix. The visibility into things like this is what makes DTrace so
remarkable.

I think there aren't many userland software interested in this. The only two
cases I can think of are databases -- which use direct I/O partly because of
this issue -- and real-time software like multimedia software -- which use
aren't so much interested in measuring it as forcing things to be preloaded
with stuff like posix_fadvise() or mlock().

I don't think DTrace is overkill either. The programmatic interface is
undocumented (but I've gotten Sun people to admit it exists -- I just have to
reverse engineer it from the existing code samples) but should be more or less
exactly what we need.

But the lowest-common-denominator of just timing read() and seeing if it took
long enough to involve either a context switch or sleeping on physical i/o
should be a pretty close approximation. The case where it would be least
accurate is when most or all of the data is actually in the cache. Then even
with a low false-positive rate detecting cache misses it'll still dominate the
true near-zero rate of cache misses.

We could mitigate that somewhat by describing it in the plan as something like

................... (... I/O fast=nnn slow=nnn)

instead of the more descriptive "physical" and "logical"

--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com Ask me about EnterpriseDB's 24x7 Postgres support!

Greg,

Gregory Stark wrote:
> I don't think DTrace is overkill either. The programmatic interface is
> undocumented (but I've gotten Sun people to admit it exists -- I just have to
> reverse engineer it from the existing code samples) but should be more or less
> exactly what we need.
>
>   
You  probably know this already. There are existing commands that use 
the programmatic interface and would provide a good starting point. Here 
are a couple:

http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/cmd/lockstat/lockstat.c
http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/cmd/dtrace/dtrace.c

One of my colleagues is in the process of putting a tutorial together 
for how to do this, so if you decided to pursue this approach and need 
assistance, please let me know.

Regards,
Robert