Re: [RFC -v3 PATCH 2/3] sched: add yield_to function

[Avi Kivity <avi@redhat.com>]

On 01/05/2011 10:40 AM, KOSAKI Motohiro wrote:
> >  On 01/05/2011 04:39 AM, KOSAKI Motohiro wrote:
> >  >  >   On 01/04/2011 08:14 AM, KOSAKI Motohiro wrote:
> >  >  >   >   Also, If pthread_cond_signal() call sys_yield_to imlicitly, we can
> >  >  >   >   avoid almost Nehalem (and other P2P cache arch) lock unfairness
> >  >  >   >   problem. (probaby creating pthread_condattr_setautoyield_np or similar
> >  >  >   >   knob is good one)
> >  >  >
> >  >  >   Often, the thread calling pthread_cond_signal() wants to continue
> >  >  >   executing, not yield.
> >  >
> >  >  Then, it doesn't work.
> >  >
> >  >  After calling pthread_cond_signal(), T1 which cond_signal caller and T2
> >  >  which waked start to GIL grab race. But usually T1 is always win because
> >  >  lock variable is in T1's cpu cache. Why kernel and userland have so much
> >  >  different result? One of a reason is glibc doesn't have any ticket lock scheme.
> >  >
> >  >  If you are interesting GIL mess and issue, please feel free to ask more.
> >
> >  I suggest looking into an explicit round-robin scheme, where each thread
> >  adds itself to a queue and an unlock wakes up the first waiter.
>
> I'm sure you haven't try your scheme. but I did. It's slow.

Won't anything with a heavily contented global/giant lock be slow?

What's the average lock hold time per thread? 10%? 50%? 90%?

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