Re: Patch: fix lock contention for HASHHDR.mutex

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Aleksander Alekseev <a(dot)alekseev(at)postgrespro(dot)ru>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Patch: fix lock contention for HASHHDR.mutex
Date: 2015-12-17 15:14:18
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On 2015-12-17 09:47:57 -0500, Robert Haas wrote:
> On Tue, Dec 15, 2015 at 7:25 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > I'd consider using a LWLock instead of a spinlock here. I've seen this
> > contended in a bunch of situations, and the queued behaviour, combined
> > with directed wakeups on the OS level, ought to improve the worst case
> > behaviour measurably.
> Amit had the idea a while back of trying to replace the HASHHDR mutex
> with something based on atomic ops. It seems hard to avoid the
> attendant A-B-A problems but maybe there's a way.

It'd really like to see it being replaced by a queuing lock
(i.e. lwlock) before we go there. And then maybe partition the freelist,
and make nentries an atomic. Just doing those might already be good
enough and should be a lot easier.


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