cvs commit: src/lib/libthr/thread thr_mutex.c src/lib/libkse/thread thr_mutex.c src/include pthread.h

[Kris Kennaway]

Daniel Eischen wrote:
> On Mon, 29 Oct 2007, Kris Kennaway wrote:
> 
>> kris        2007-10-29 21:01:47 UTC
>>
>>  FreeBSD src repository
>>
>>  Modified files:
>>    lib/libthr/thread    thr_mutex.c
>>    lib/libkse/thread    thr_mutex.c
>>    include              pthread.h
>>  Log:
>>  Add a new "non-portable" mutex type, PTHREAD_MUTEX_ADAPTIVE_NP.  This
>>  is also implemented in glibc and is used by a number of existing
>>  applications (mysql, firefox, etc).
>>
>>  This mutex type is a default mutex with the additional property that
>>  it spins briefly when attempting to acquire a contested lock, doing
>>  trylock operations in userland before entering the kernel to block if
>>  eventually unsuccessful.
>>
>>  The expectation is that applications requesting this mutex type know
>>  that the mutex is likely to be only held for very brief periods, so it
>>  is faster to spin in userland and probably succeed in acquiring the
>>  mutex, than to enter the kernel and sleep, only to be woken up almost
>>  immediately.  This can help significantly in certain cases when
>>  pthread mutexes are heavily contended and held for brief durations
>>  (such as mysql).
>>
>>  Spin up to 200 times before entering the kernel, which represents only
>>  a few us on modern CPUs.  No performance degradation was observed with
>>  this value and it is sufficient to avoid a large performance drop in
>>  mysql performance in the heavily contended pthread mutex case.
>>
>>  The libkse implementation is a NOP.
> 
> The libkse implementation already spins for a bit.  The default
> number of spins is 500.

OK, cool.

> I'm not sure that another mutex type is warranted, the default
> mutex implementation should be adaptive I think.

The point being that certain existing applications already know about 
this mutex name and will use it automatically when it exists.

I am a bit wary of making this the default type though.  The algorithm 
is a pessimization when the conditions described above are not true.

Kris