Re: Use fadvise in wal replay

> On 21 Jun 2022, at 20:52, Pavel Borisov <pashkin.elfe@gmail.com> wrote:
>
> > On 21 Jun 2022, at 16:59, Jakub Wartak <jakub.wartak@tomtom.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
prefetchbuffer) 
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
unchangedno matter what. 
>
> I became convinced that if I read something long, OS does necessary speedups automatically (which is also in
agreementwith 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
surelyworks :) The question is how to use it optimally. 

Best regards, Andrey Borodin.