Re: Use fadvise in wal replay

> On 21 Jun 2022, at 20:52, Pavel Borisov  wrote:
> 
> > On 21 Jun 2022, at 16:59, Jakub Wartak  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.