Re: Once again: test_and_set for CPUs w/o LL/SC

[Johannes Stezenbach <js@convergence.de>]

On Thu, Oct 17, 2002 at 05:32:03PM +0400, Gleb O. Raiko wrote:
> Johannes Stezenbach wrote:
> > 
> > On Thu, Oct 17, 2002 at 02:02:35PM +0200, Maciej W. Rozycki wrote:
> > > On Thu, 17 Oct 2002, Gleb O. Raiko wrote:
> > >
> > > > Implement new sysmips then.
> > >
> > >  I'm not sure if that's a good idea.  Glibc alone uses test_and_set(),
> > > exchange_and_add(), atomic_add() and compare_and_swap().  Do you want a
> > > separate syscall for each of these functions?  I think the ll/sc emulation
> > > may be the best solution after all.  At least it's most flexible and not
> > > much slower if at all.
> > 
> > Depends on your usage pattern. E.g. we don't run software that uses
> > atomicity.h (i.e. no C++ code), but heavily use pthread_mutex_lock() etc.
> > The few uses of atomicity.h internal to glibc don't warrant
> > any optimizations. So, if the beql-Method would not exist, I would
> > consider implementing a new sysmips for compare_and_swap().
> 
> I didn't look at newer glibc sources (read: greater than 2.0.6), so the
> question. Why  is the difference between compare_and_swap and
> test_and_set so huge that it eats an exception penalty? ;-)

It is not. I wrote:
  ... But with LL/SC glibc can use compare-and-swap
  which enables a more efficient linux-threads mutex implementation.

This is what makes the difference, at least for glibc-2.2.5. Just
grep for HAS_COMPARE_AND_SWAP in your linuxthreads sources.

Current glibc from CVS (both HEAD an 2.2 branch) doesn't use sysmips
anymore, they rely on LL/SC (emulated or not).


Regards,
Johannes