Re: [HACKERS] Fix performance degradation of contended LWLock on NUMA

From: Юрий Соколов <funny(dot)falcon(at)gmail(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: jesper(dot)pedersen(at)redhat(dot)com, PostgreSQL-Dev <pgsql-hackers(at)postgresql(dot)org>, pgsql-hackers-owner(at)postgresql(dot)org
Subject: Re: [HACKERS] Fix performance degradation of contended LWLock on NUMA
Date: 2017-11-27 12:41:46
Message-ID: CAL-rCA3MTrRm3dMJKheTOYz=y+5ub3ithM+-UOggho2USf6ung@mail.gmail.com
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2017-11-26 19:51 GMT+03:00 Юрий Соколов <funny(dot)falcon(at)gmail(dot)com>:
>
> 2017-11-06 18:07 GMT+03:00 Sokolov Yura <funny(dot)falcon(at)postgrespro(dot)ru>:
> >
> > On 2017-10-20 11:54, Sokolov Yura wrote:
> >>
> >> Hello,
> >>
> >> On 2017-10-19 19:46, Andres Freund wrote:
> >>>
> >>> On 2017-10-19 14:36:56 +0300, Sokolov Yura wrote:
> >>>>
> >>>> > > + init_local_spin_delay(&delayStatus);
> >>>> >
> >>>> > The way you moved this around has the disadvantage that we now do
this -
> >>>> > a number of writes - even in the very common case where the lwlock
can
> >>>> > be acquired directly.
> >>>>
> >>>> Excuse me, I don't understand fine.
> >>>> Do you complain against init_local_spin_delay placed here?
> >>>
> >>>
> >>> Yes.
> >>
> >>
> >> I could place it before perform_spin_delay under `if (!spin_inited)`
if you
> >> think it is absolutely must.
> >
> >
> > I looked at assembly, and remembered, that last commit simplifies
> > `init_local_spin_delay` to just two-three writes of zeroes (looks
> > like compiler combines 2*4byte write into 1*8 write). Compared to
> > code around (especially in LWLockAcquire itself), this overhead
> > is negligible.
> >
> > Though, I found that there is benefit in calling LWLockAttemptLockOnce
> > before entering loop with calls to LWLockAttemptLockOrQueue in the
> > LWLockAcquire (in there is not much contention). And this way, `inline`
> > decorator for LWLockAttemptLockOrQueue could be omitted. Given, clang
> > doesn't want to inline this function, it could be the best way.
>
> In attach version with LWLockAcquireOnce called before entering loop
> in LWLockAcquire.
>

Oh... there were stupid error in previos file.
Attached fixed version.

Attachment Content-Type Size
lwlock_v6.patch.gz application/x-gzip 9.8 KB

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