Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks - base implementation

From: Jan Beulich
Date: Wed Jun 30 2010 - 09:21:18 EST

>>> On 30.06.10 at 14:53, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> On 06/30/2010 01:52 PM, Jan Beulich wrote:
>> I fail to see that: Depending on the hypervisor's capabilities, the
>> two main functions could be much smaller (potentially there wouldn't
>> even be a need for the unlock hook in some cases),
> What mechanism are you envisaging in that case?

A simple yield is better than not doing anything at all.

>> The list really juts is needed to not pointlessly tickle CPUs that
>> won't own the just released lock next anyway (or would own
>> it, but meanwhile went for another one where they also decided
>> to go into polling mode).
> Did you measure that it was a particularly common case which was worth
> optimising for?

I didn't measure this particular case. But since the main problem
with ticket locks is when (host) CPUs are overcommitted, it
certainly is a bad idea to create even more load on the host than
there already is (the more that these are bursts).


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