Re: [PATCH -v6][RFC]: mutex: implement adaptive spinning

[Gregory Haskins]

Ingo Molnar wrote:
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>   
>> * WOW *
>>     
>
> WOW indeed - and i can see a similar _brutal_ speedup on two separate 
> 16-way boxes as well:
>
>   16 CPUs, running 128 parallel test-tasks.
>
>   NO_OWNER_SPIN:
>   avg ops/sec:               281595
>
>   OWNER_SPIN:
>   avg ops/sec:               524791
>
> Da Killer!
>   

This jives with our findings back when we first looked at this
(200%-300% speedups in most benchmarks), so this is excellent that it is
yielding boosts here as well.

> Look at the performance counter stats:
>
>   
>>    12098.324578  task clock ticks     (msecs)
>>
>>            1081  CPU migrations       (events)
>>            7102  context switches     (events)
>>            2763  pagefaults           (events)
>>     
>
>   
>>    22280.283224  task clock ticks     (msecs)
>>
>>             117  CPU migrations       (events)
>>            5711  context switches     (events)
>>            2781  pagefaults           (events)
>>     
>
> We were able to spend twice as much CPU time and efficiently so - and we 
> did about 10% of the cross-CPU migrations as before (!).
>
> My (wild) guess is that the biggest speedup factor was perhaps this little 
> trick:
>
> +               if (need_resched())
> +                       break;
>
> this allows the spin-mutex to only waste CPU time if there's no work 
> around on that CPU. (i.e. if there's no other task that wants to run) The 
> moment there's some other task, we context-switch to it.
>   
Well, IIUC thats only true if the other task happens to preempt current,
which may not always be the case, right?  For instance, if current still
has timeslice left, etc.  I think the primary difference is actually the
reduction in the ctx switch rate, but its hard to say without looking at
detailed traces and more stats.  Either way, woohoo!

-Greg

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