The only one of those that ABSOLUTELY blocks copy-in/copy-out and the more-or-less equivalent optimisations is VOLATILE. ASYNCHRONOUS does the least blocking, and is therefore the most efficient.

Ugh. I have thought of a case where even VOLATILE doesn't block copy-in/copy-out, but it's irrelevant to this debate. I am also pretty sure that I can construct an example where TARGET isn't enough to make MPI non-blocking transfers safe, but would need to check that with the standard very carefully. Regards, Nick Maclaren, University of Cambridge Computing Service, New Museums Site, Pembroke Street, Cambridge CB2 3QH, England. Email: [email protected] Tel.: +44 1223 334761 Fax: +44 1223 334679