On Thu, 4 Feb 1999, Ignatios Souvatzis wrote:

> > |	CACHESYNC_GLOBAL
> > |
> > |	as above, but for multi-threaded applications.
> > 
> > This is identical to CACHESYNC for single CPU machines, or at least
> > architectures. However: cgd mentioned (elsewhere) nobody should want this,
> > and if yes, that this could be done by STOREBAR (see below),
> > application-internal synchronization, and CACHESYNC in all threads. Opinions?

On a multiprocessor machine CACHESYNC_GLOBAL would need to be used in
situations where it is necessary to tell other processors to do a
CACHESYNC because there are no convenient synchronization points.  For
instance, if you have the JIT compiler running on once CPU and the other
CPUs are executing already compiled code, when the JIT compiler completes
compilation of a code block it will need to inform the other CPUs to flush
the appropriate sections of their caches.  Adding some way to tell all the
other CPUs to issue a CACHESYNC is problematical and would affect
performance, whereas a CACHESYNC_GLOBAL would issue cross-call interrupts
or equivalent to all CPUs to flush their caches immediately.

But this is a rather obscure application, and since there really aren't
that many JIT compilers, I don't expect it to be used very often.  OTOH,
the STOREBAR and LOOKASIDE would probably be used much more often for
spinlocks, etc.

=========================================================================
Eduardo Horvath				eeh@one-o.com
	"I need to find a pithy new quote." -- me