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

Avi Kivity
2011-01-05T08:35:44Z

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.

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