Re: RFC: Ideal Adaptive Spinning Conditions

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

Avi Kivity wrote:
> On 04/01/2010 06:54 PM, Darren Hart wrote:
>>> A lock(); unlock(); loop spends most of its time with the lock held 
>>> or contended.  Can you something like this:
>>>
>>>
>>>    lock();
>>>    for (i = 0; i < 1000; ++i)
>>>         asm volatile ("" : : : "memory");
>>>    unlock();
>>>    for (i = 0; i < 10000; ++i)
>>>         asm volatile ("" : : : "memory");
>>
>>
>>
>> Great idea. I'll be doing a more rigorous investigation on this of 
>> course, but I thought I'd share the results of just dumping this into 
>> the testcase:
>>
>> # ./futex_lock -i10000000
>> futex_lock: Measure FUTEX_LOCK operations per second
>>     Arguments: iterations=10000000 threads=256 adaptive=0
>> Result: 420 Kiter/s
>> lock calls:      9999872
>> lock syscalls:   665824 (6.66%)
>> unlock calls:    9999872
>> unlock syscalls: 861240 (8.61%)
>>
>> # ./futex_lock -a -i10000000
>> futex_lock: Measure FUTEX_LOCK operations per second
>>     Arguments: iterations=10000000 threads=256 adaptive=1
>> Result: 426 Kiter/s
>> lock calls:      9999872
>> lock syscalls:   558787 (5.59%)
>> unlock calls:    9999872
>> unlock syscalls: 603412 (6.03%)
>>
>> This is the first time I've seen adaptive locking have an advantage! 
>> The second set of runs showed a slightly greater advantage. Note that 
>> this was still with spinners being limited to one.
> 
> Question - do all threads finish at the same time, or wildly different 
> times?

I'm not sure, I can add some fairness metrics to the test that will help 
characterize how that's working. My suspicion is that there will be 
several threads that don't make any progress until the very end - since 
adaptive spinning is an "unfair" locking technique.

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