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

[Kris Kennaway]

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