Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

[Darren Hart <dvhltc@us.ibm.com>]

Peter Zijlstra wrote:
> On Tue, 2010-04-06 at 07:47 -0700, Ulrich Drepper wrote:
>> On Tue, Apr 6, 2010 at 01:48, Peter Zijlstra <peterz@infradead.org>
>> wrote:
>>>  try
>>>  spin
>>>  try
>>>  syscall
>> This is available for a long time in the mutex implementation
>> (PTHREAD_MUTEX_ADAPTIVE_NP mutex type).  It hasn't show much
>> improvement if any.  There were some people demanding this support for
>> as far as I know they are not using it now.  This is adaptive
>> spinning, learning from previous calls how long to wait.  But it's
>> still unguided.  There is no way to get information like "the owner
>> has been descheduled". 
> 
> That's where the FUTEX_LOCK thing comes in, it does all those, the above
> was a single spin loop to amortize the syscall overhead.
> 
> I wouldn't make it any more complex than a single pause ins, syscalls
> are terribly cheap these days.

And yet they still seem to have a real impact on the futex_lock 
benchmark. Perhaps I am just still looking at pathological cases, but 
there is a strong correlation between high syscall counts and really low 
iterations per second. Granted this also correlates with lock 
contention. However, when using the same period and duty-cycle I find 
that a locking mechanism that makes significantly fewer syscalls also 
significantly outperforms one that makes more. Kind of handwavy stilly, 
I'll have more numbers this afternoon.

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team