Re: Patch: fix lock contention for HASHHDR.mutex

On Thu, Dec 17, 2015 at 8:44 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> 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.
>

makes sense to me, but I think we should as well try the Group leader idea
used for ProcArrayLock optimisation as during those tests, I found that
it gives better results as compare to partitioning.

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