Re: Poor condvar performance

On Wed, 2003-11-26 at 17:53, Jamie Lokier wrote:
> Ulrich Drepper wrote:
> > Requeue cannot work with pshared.  requeue needs the virtual address of
> > the mutex but this cannot be determined when using pshared since it
> > might be different in different processes.  It would be necessary to
> > have a process-local part of the condvar structure to remember the mutex
> > address for each process separately.  There is no such functionality
> > which is sufficiently fast.
> 
> (I haven't looked at the NPTL code, I just know the futex code well).
> 
> Why do you need to remember the mutex address in each process, if it's
> in shared memory?  Is it because the requeue operation would have to
> move a waiter from a shared memory futex to a process-local memory futex?
> 
> -- Jamie

Ok I'm now getting out of my depth, but doesn't using a pshared condvar
imply the associated mutex is also pshared? I understand that a single
mutex can be associated with many condvars, but to associate a condvar
with different local mutexes in different processes doesn't seem to make
sense.

Essentially then the current pshared NPTL condvars are unusable, at
least on UP? What alternatives are there to condvars? Using the futex
API directly?

Regards

Luke.