Re: critical section

["Ye Liu" <yliu@tibco.com>]

You suggest that I leave it to the system decide whether "spin" or "yield". Is
there a function in pthread to decide how many processors the system has?

--ye

Eli Ofenstein wrote:

> > -----Original Message-----
> > From: Scott McCaskill [ mailto:scott@magruder.org ]
> > Sent: Tuesday, July 31, 2001 5:52 PM
> > To: Ye Liu
> > Cc: pthreads-win32@sourceware.cygnus.com
> > Subject: Re: critical section
> >
> >
> > ----- Original Message -----
> > From: "Ye Liu" <yliu@tibco.com>
> > To: "Scott McCaskill" <scott@magruder.org>
> > Cc: <pthreads-win32@sourceware.cygnus.com>
> > Sent: Tuesday, July 31, 2001 5:36 PM
> > Subject: Re: critical section
> >
> >
> > > In the book of "Programming with POSIX Threads",  the
> > author metioned
> > >
> > > "You cannot lock a mutex when the calling thread already
> > has that mutex
> > locked."
> > >
> > > My previous understanding is "a mutex cannot be locked twice", which
> > obviously
> > > is wrong.
> > >
> > > If I use a non-recursive mutex, when a thread try to lock
> > the mutex which
> > is
> > > already locked by another one, what happens to the calling
> > thread? Spin or
> > > yield?
> > >
> >
> > I don't know for sure, but I would expect it to yield.  It
> > seems like the
> > spinning that your code is doing would be purely wasteful unless the
> > spinning thread and the mutex-holding thread are on different
> > processors.
> > Can you give us a better idea of what you're trying to accomplish that
> > pthread_mutex won't do for you?
>
> That's a big "unless" :)
>
> In many implementations, a hybrid approach is used in which the thread spins
> for a period of time before yielding, in order to allow fast critical
> sections to complete.  Additionally, implementations are often aware of the
> CPU count of the system, and will adopt an immediate-yield policy on single
> CPU systems.
>

--
Ye Liu
Tel(O) 650-846-5228