Re: [PATCH 4/4, v2] x86: enlightenment for ticket spin locks - improve yield behavior on Xen

From: Jan Beulich
Date: Wed Jun 30 2010 - 07:25:09 EST

>>> On 30.06.10 at 12:10, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> On 06/30/2010 10:49 AM, Jan Beulich wrote:
>>>>> On 30.06.10 at 10:11, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>> On Tue, 2010-06-29 at 15:35 +0100, Jan Beulich wrote:
>>>> The (only) additional overhead this introduces for native execution is
>>>> the writing of the owning CPU in the lock acquire paths.
>>> Uhm, and growing the size of spinlock_t to 6 (or 8 bytes when aligned)
>>> bytes when NR_CPUS>256.
>> Indeed, I should have mentioned that. Will do so in an eventual
>> next version.
> Rather than increasing the lock size, why not just disable the
> enlightenment if the number of (possible) cpus is > 256? I don't think
> a VM will ever have that many cpus, so it will only apply in the case of
> booting the kernel on large physical machine.

While that would be an option, I think the decision to go either
way should really be left to the user (after all that's why there
is a config option in the first place).


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