linux-kernel - Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin with irqs disable

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Date:	Mon, 07 Jul 2008 08:56:13 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <clameter@....com>,
	Petr Tesarik <ptesarik@...e.cz>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin
 with irqs disable

Nick Piggin wrote:
> Although, you wouldn't need to oversubscribe physical CPUs to hit
> suboptimal behaviour.
>
> Basically, I just ask for performance improvement to be measured
> with some "realistic" configuration, then it should be easier to
> justify.

Overcommitting cpus system-wide is pretty common.  A use-case is 
consolidating a pile of underused non-performance-critical servers into 
one physical machine with a lot of VMs.  It doesn't take much load to 
cause a dhcp request to the dhcp server to steal a cpu away from the 
mysql server while it holds a lock...

I'll mail out my patches so we can have a more concrete discussion shortly.

    J
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