Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets - bisected

[Andi Kleen]

Mike Galbraith <efault@xxxxxx> writes:
>
> The only reason I can think of why pgbench might suffer is postgres's
> userspace spinlocks.  If you always look for an idle core, you improve
> the odds that the wakeup won't preempt a lock holder, sending others
> into a long spin.

User space spinlocks like this unfortunately have a tendency to break
with all kinds of scheduler changes. We've seen this frequently too
with other users. The best bet currently is to use the real time
scheduler, but with various tweaks to get its overhead down.

Ultimatively the problem is that user space spinlocks with CPU
oversubcription is a very unstable setup and small changes can
easily disturb it.

Just using futex is unfortunately not the answer either.

-Andi

-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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