Re: Move PinBuffer and UnpinBuffer to atomics

On Tue, Apr 5, 2016 at 8:15 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> On 2016-04-05 17:36:49 +0300, Alexander Korotkov wrote:
> > Could the reason be that we're increasing concurrency for LWLock state
> > atomic variable by placing queue spinlock there?
>
> Don't think so, it's the same cache-line either way.
>
> > But I wonder why this could happen during "pgbench -S", because it
doesn't
> > seem to have high traffic of exclusive LWLocks.
>
> Yea, that confuses me too. I suspect there's some mis-aligned
> datastructures somewhere. It's hard to investigate such things without
> access to hardware.
>

This fluctuation started appearing after commit 6150a1b0 which we have
discussed in another thread [1] and a colleague of mine is working on to
write a patch to try to revert it on current HEAD and then see the results.

> (FWIW, I'm working on getting pinunpin committed)
>

Good to know, but I am slightly worried that it will make the problem
harder to detect as it will reduce the reproducibility. I understand that
we are running short of time and committing this patch is important, so we
should proceed with it as this is not a problem of this patch. After this
patch gets committed, we always need to revert it locally to narrow down
the problem due to commit 6150a1b0.

[1] -
http://www.postgresql.org/message-id/CAA4eK1+ZeB8PMwwktf+3bRS0Pt4Ux6Rs6Aom0uip8c6shJWmyg@mail.gmail.com

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com