From: | Andrey Borodin <x4mmm(at)yandex-team(dot)ru> |
---|---|
To: | Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> |
Cc: | Jakub Wartak <jakub(dot)wartak(at)tomtom(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Kirill Reshke <reshke(at)double(dot)cloud>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: Use fadvise in wal replay |
Date: | 2022-06-22 10:07:07 |
Message-ID: | 979BDEE2-49C1-4420-8578-9182BC2DE875@yandex-team.ru |
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Lists: | pgsql-hackers |
> On 21 Jun 2022, at 20:52, Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
>
> > On 21 Jun 2022, at 16:59, Jakub Wartak <jakub(dot)wartak(at)tomtom(dot)com> wrote:
> Oh, wow, your benchmarks show really impressive improvement.
>
> FWIW I was trying to speedup long sequential file reads in Postgres using fadvise hints. I've found no detectable improvements.
> Then I've written 1Mb - 1Gb sequential read test with both fadvise POSIX_FADV_WILLNEED and POSIX_FADV_SEQUENTIAL in Linux.
Did you drop caches?
> The only improvement I've found was
>
> 1. when the size of read was around several Mb and fadvise len also around several Mb.
> 2. when before fdavice and the first read there was a delay (which was supposedly used by OS for reading into prefetch buffer)
That's the case of startup process: you read a xlog page, then redo records from this page.
> 3. If I read sequential blocks i saw speedup only on first ones. Overall read speed of say 1Gb file remained unchanged no matter what.
>
> I became convinced that if I read something long, OS does necessary speedups automatically (which is also in agreement with fadvise manual/code comments).
> Could you please elaborate how have you got the results with that big difference? (Though I don't against fadvise usage, at worst it is expected to be useless).
FWIW we with Kirill observed drastically reduced lag on a production server when running patched version. Fidvise surely works :) The question is how to use it optimally.
Best regards, Andrey Borodin.
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