cvs commit: src/lib/libthr/thread thr_pspinlock.c

[David Xu]

Kris Kennaway wrote:
> Jason Evans wrote:
> 
>> Kris Kennaway wrote:
>>
>>> David Xu wrote:
>>>
>>>>   FreeBSD src repository
>>>>
>>>>   Modified files:
>>>>     lib/libthr/thread    thr_pspinlock.c   Log:
>>>>   Reverse the logic of UP and SMP.
>>>>     Submitted by: jasone
>>>>     Revision  Changes    Path
>>>>   1.6       +1 -1      src/lib/libthr/thread/thr_pspinlock.c
>>>
>>>
>>> Are there any common applications that use this?
>>
>>
>> It's worth mentioning that this change, although correct, does not 
>> make a measurable performance difference for the tests I was running 
>> when I found the bug.  It is possible that making the spinlocks 
>> adaptive would help, but I didn't look into this.
>>
>> (I was working on malloc performance enhancements that have turned out 
>> very nicely, but in the end I had to switch to hand-rolled "spin" 
>> mutexes that eventually convert to blocking, in order to avoid the 
>> possibility of unrecoverable priority inversion.)
> 
> 
> BTW I am looking at adding a non-portable (sort of) pthread mutex type 
> that spins for a while when the lock is held, before blocking.  This is 
> sometimes good for performance when the pthread mutex is highly 
> contended but held for short periods of time, and in fact Linux has such 
> a mutex that is used by mysql with performance benefits.
> 
> The real fix would be to make them adaptive in the same way as kernel 
> mutexes (spin as long as the lock holder is running), but there is 
> currently no easy way for userland to peer into the kernel to check the 
> other thread's state.
> 
> Kris
> 

I have a patch to adaptively spin in userland, kernel is not involved.

http://people.freebsd.org/~davidxu/patch/libthr_spin.patch

I have not seen visible performance improvement in some tests,
unlike kernel code, many user application code do not care parallel 
performance, they only care exclusive accessing to a resource, you can
call them badly written code, so spinning is not always useful.
However, with this patch, Sun's mutex ping-pong program's performance
is massively improved, try it.

Regards,
David Xu