linux-kernel - Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)

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Date:	Wed, 12 Mar 2008 10:42:43 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Nicholas Miell <nmiell@...cast.net>
Cc:	Cyrus Massoumi <cyrusm@....net>, "Molnar, Ingo" <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)

On Wednesday 12 March 2008 10:12, Nicholas Miell wrote:
> On Tue, 2008-03-11 at 22:34 +0100, Cyrus Massoumi wrote:

> Piggin said the following:
> > The problem with MySQL contention means that if the scheduler
> > unluckily chooses to deschedule a lock holder, then you can get
> > idle time building up on other cores and you can get context switch
> > cascades as things all pile up onto this heavily contended lock. As
> > such, it means MySQL is not an ideal candidate for looking at
> > performance behaviour. I discounted the relatively worse scaling of
> > MySQL with 2.6.25-rc (than 2.6.22) as such an effect.
>
> which I interpreted to mean that MySQL performs worse on 2.6.23+ than on
> 2.6.22 but for some reason this doesn't matter.

I didn't try 2.6.23, which I think has bigger problems due to new
CFS code. 2.6.25-rc fixes that, but yes in general I think the MySQL
performance profile is worse on later kernels: while it has a
very slightly higher peak performance, it drops off in performance
more quickly after that peak. I initially wasn't too worried about
it, but seeing as postgresql has a similar problem, I've made the
scheduler developers aware of it.

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