An idea of remove MUTEX_WAKE_ALL

Daniel Eischen deischen at
Tue Jan 3 11:26:08 PST 2006

On Tue, 3 Jan 2006, John Baldwin wrote:

> On Sunday 01 January 2006 02:21 am, prime wrote:
> > Hi hackers,
> >    I have an idea about remove the kernel option MUTEX_WAKE_ALL.
> >    When we unlock the mutex(in _mtx_unlock_sleep),we can directly
> > give the lock to the first thread waiting on the turnstile.And a
> > thread gets the mutex after he returned from turnstile_wait so he
> > can simply jump out the _obtain_lock loop in _mtx_lock_sleep.
> > This makes a mutex always be owned by a thread when there are threads
> > waiting on the turnstile,so priority inheritance can work now.
> >    This idea need only a few changes in kern/kern_mutex.c .But when
> > NO_ADAPTIVE_MUTEXS not set,it makes threads that spinning on other CPU
> > to get the mutex have to spin for a long time,and this makes the short
> > term mutex more expensive(maybe should use spin mutex instead).
> >
> > What do think about the idea? Thanks.
> Sun actually found that the performance was better when you did MUTEX_WAKE_ALL
> because once you woke up N threads, if they don't all resume at once then
> they will acquire the lock in sequence and the lock acquires and releaes will
> all be simple ones rather than all being the complicated contested case.
> There are more details in _Solaris Internals_.

Yes, but doesn't this partly rely on having the threads spin(*)
for a bit if the current lock owner is running on another CPU?
Do we currently do that?

(*) No, I am not referring to spin mutexes.


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