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

пт, 30 нояб. 2018 г., 19:21 Dmitry Dolgov 9erthalion6(at)gmail(dot)com:

> > On Sat, Nov 10, 2018 at 8:37 PM Andres Freund <andres(at)anarazel(dot)de>
> wrote:
> >
> > On 2018-11-10 20:18:33 +0100, Dmitry Dolgov wrote:
> > > > On Mon, 2 Jul 2018 at 15:54, Jesper Pedersen <
> jesper(dot)pedersen(at)redhat(dot)com> wrote:
> > > >
> > > > The patch from November 27, 2017 still applies (with hunks),
> > > >
> > > > https://commitfest.postgresql.org/18/1166/
> > > >
> > > > passes "make check-world" and shows performance improvements.
> > > >
> > > > Keeping it in "Ready for Committer".
> > >
> > > Looks like for some reason this patch is failing to attract
> committers, any
> > > idea why? One of the plausible explanations for me is that the patch
> requires
> > > some more intensive benchmarking of different workloads and types of
> lock
> > > contention to make everyone more confident about it.
> >
> > Personally it's twofold:
> >
> > 1) It changes a lot of things, more than I think are strictly
> > necessary to achieve the goal.
> >
> > 2) While clearly the retry logic is not necessary anymore (it was
> > introduced when wait-queue was protected by a separate spinlock, which
> > could not atomically manipulated together with the lock's state),
> > there's reasons why it would be advantageous to keep: My original
> > patch for changing lwlocks to atomics, used lock xadd / fetch_add
> > to acquire shared locks (just incrementing the #shared bits after an
> > unlocked check) - obviously that can cause superfluous failures for
> > concurrent lock releases. Having the retry logic around can make
> > that safe.
> >
> > Using lock xadd to acquire shared locks turns out to be considerably
> > more efficient - most locks where the lock state is contended (rather
> > than just having processes wait), tend to have a very large fraction
> > of shared lockers. And being able to do such a lock acquisition on a
> > conteded cacheline with just a single locked operation, commonly
> > without retries, is quite beneficial.
>
> Due to lack of response and taking into account this commentary, I'm
> marking
> this patch as "Returned with feedback", but hopefully I can pick it up
> later to
> improve.
>

Good luck.

>