Re: Using read stream in autoprewarm - Mailing list pgsql-hackers

On Wed, 27 Nov 2024 at 19:20, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:
>
> Hi,
>
> Thank you for looking into this!
>
> On Wed, 27 Nov 2024 at 16:50, Matheus Alcantara <mths.dev@pm.me> wrote:
> > I've executed the same test of 5 databases with each of them having 1 table of
> > 3GB of size and I've got very similar results.
> >
> > I've also tested using a single database with 4 tables with ~60GB of size and
> > the results compared with master was more closer but still an improvement. Note
> > that I've also increased the default shared_buffers to 7GB to see how it works
> > with large buffer pools.
> >   - patched: 5.4259 s
> >   - master: 5.53186 s
>
> Thanks for the testing.
>
> > Not to much to say about the code, I'm currently learning more about the read
> > stream API and Postgresql hacking itself. Just some minor points and questions
> > about the patches.
> >
> >
> > v2-0002-Count-free-buffers-at-the-start-of-the-autoprewar.patch
> > --- a/src/backend/storage/buffer/freelist.c
> > +/*
> > + * get_number_of_free_buffers -- a lockless way to get the number of free
> > + *                                                              buffers in buffer pool.
> > + *
> > + * Note that result continuosly changes as free buffers are moved out by other
> > + * operations.
> > + */
> > +int
> > +get_number_of_free_buffers(void)
> >
> > typo on continuosly -> continuously
>
> Done.
>
> > v2-0001-Use-read-stream-in-autoprewarm.patch
> > +       bool       *rs_have_free_buffer = per_buffer_data;
> > +
> > +
> > +       *rs_have_free_buffer = true;
> > +
> >
> > Not sure if I understand why this variable is needed, it seems that it is only
> > written and never read? Just as comparison, the block_range_read_stream_cb
> > callback used on pg_prewarm seems to not use the per_buffer_data parameter.
>
> Actually, it is read in the main loop of the
> autoprewarm_database_main() function:
>
>         /* There are no free buffers left in shared buffers, break the loop. */
>         else if (!(*rs_have_free_buffer))
>             break;
>
> apw_read_stream_next_block() callback function sets
> rs_have_free_buffer's value to false when there is no free buffer left
> in the shared buffers. And the code above terminates the main loop in
> the autoprewarm_database_main() function when it is set to false.
>
> block_range_read_stream_cb() callback is used when the callback only
> needs to loop over the block numbers. However, for the autoprewarm
> case; the callback function needs to do additional checks so another
> callback and the use of this variable are required.
>
> v3 is attached.
>
> --
> Regards,
> Nazir Bilal Yavuz
> Microsoft

Hi!

> + old_blk = &(p->block_info[p->pos - 1]);
> + cur_blk = &(p->block_info[p->pos]);
Should we Assert(p->pos > 0 && p->pos < *something*)

Patch tested with no regression.


-- 
Best regards,
Kirill Reshke