Subject: Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)
From: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
To: Nicholas Miell <nmiell@comcast.net>
Cc: Cyrus Massoumi <cyrusm@gmx.net>, Nick Piggin <nickpiggin@yahoo.com.au>,
       "Molnar, Ingo" <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <1205277132.12854.19.camel@entropy>
References: <200803111749.29143.nickpiggin@yahoo.com.au>
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	 <1205277132.12854.19.camel@entropy>
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Date: Wed, 12 Mar 2008 17:00:55 +0800
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On Tue, 2008-03-11 at 16:12 -0700, Nicholas Miell wrote:
> On Tue, 2008-03-11 at 22:34 +0100, Cyrus Massoumi wrote:
> > Nicholas Miell wrote:
> >
> > > (Also, ignoring MySQL because it's a terrible piece of software at least
> > > when regarding it's scalability is a bad idea. It's the M in LAMP, it
> > > has a huge user base, and FreeBSD manages to outperform Linux with the
> > > same unscalable piece of software.)
> > 
> > Did you actually see this? 
> > http://www.kernel.org/pub/linux/kernel/people/npiggin/sysbench/
> > 
> > FreeBSD does not outperform Linux, it's actually a bit faster according 
> > to Nick's tests.
> 
> I am aware of those results, but in the mail I was responding to, Nick
> 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.
That's because 2.6.23-rc lack the cache hot feature of scheduler. Ingo fixed
it in 2.6.24-rc.

-yanmin


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