Re: Better LWLocks with compare-and-swap (9.4)

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Em 13-05-2013 09:50, Heikki Linnakangas escreveu:
> I've been working on-and-off on the WAL-insert scaling patch. It's
> in pretty good shape now, and I'll post it shortly, but one thing I
> noticed is that it benefits a lot from using an atomic
> compare-and-swap instruction for the contention-critical part.
>
> I realized that we could also use compare-and-swap to make LWLocks
> scale better. The LWLock struct is too large to compare-and-swap
> atomically, but we can still use CAS to increment/decrement the
> shared/exclusive counters, when there's no need to manipulate the
> wait queue. That would help with workloads where you have a lot of
> CPUs, and a lot of backends need to acquire the same lwlock in
> shared mode, but there's no real contention (ie. few exclusive
> lockers).
>
> pgbench -S is such a workload. With 9.3beta1, I'm seeing this
> profile, when I run "pgbench -S -c64 -j64 -T60 -M prepared" on a
> 32-core Linux machine:
>
> - 64.09% postgres postgres [.] tas - tas - 99.83%
> s_lock - 53.22% LWLockAcquire + 99.87% GetSnapshotData - 46.78%
> LWLockRelease GetSnapshotData + GetTransactionSnapshot + 2.97%
> postgres postgres [.] tas + 1.53% postgres
> libc-2.13.so [.] 0x119873 + 1.44% postgres postgres
> [.] GetSnapshotData + 1.29% postgres [kernel.kallsyms] [k]
> arch_local_irq_enable + 1.18% postgres postgres [.]
> AllocSetAlloc ...

I'd like to test this here but I couldn't reproduce that perf output
here in a 64-core or 24-core machines, could you post the changes to
postgresql.conf and the perf arguments that you used?

Thanks!

[]s
- --
Dickson S. Guedes
mail/xmpp: guedes(at)guedesoft(dot)net - skype: guediz
http://guedesoft.net - http://www.postgresql.org.br
http://github.net/guedes - twitter: @guediz
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