From owner-sc22wg5@open-std.org Thu Jan 22 18:08:20 2009 Return-Path: X-Original-To: sc22wg5-dom7 Delivered-To: sc22wg5-dom7@www2.open-std.org Received: by www2.open-std.org (Postfix, from userid 521) id 54863CA5FE6; Thu, 22 Jan 2009 18:08:20 +0100 (CET) X-Original-To: sc22wg5@open-std.org Delivered-To: sc22wg5@open-std.org Received: from mail1.cray.com (mail1.cray.com [136.162.0.111]) by www2.open-std.org (Postfix) with ESMTP id 64D47CA3439 for ; Thu, 22 Jan 2009 18:08:17 +0100 (CET) Received: from beaver.us.cray.com (beaver.us.cray.com [172.30.74.51]) by mail1.cray.com (8.13.6/8.13.3/gw-5323) with ESMTP id n0MH83TM004316 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK); Thu, 22 Jan 2009 11:08:05 -0600 (CST) Received: from CFEXFE01.us.cray.com (cfexfe01.us.cray.com [172.30.74.93]) by beaver.us.cray.com (8.13.8/8.13.3/hub-5273) with ESMTP id n0MH82so019635; Thu, 22 Jan 2009 11:08:02 -0600 Received: from mh-dhcp-172-31-16-180.us.cray.com ([172.31.16.180]) by CFEXFE01.us.cray.com over TLS secured channel with Microsoft SMTPSVC(6.0.3790.3959); Thu, 22 Jan 2009 11:08:01 -0600 Message-ID: <4978A8A0.8090407@cray.com> Date: Thu, 22 Jan 2009 11:10:56 -0600 From: Bill Long Reply-To: longb@cray.com Organization: Cray Inc. User-Agent: Thunderbird 2.0.0.9 (Macintosh/20071031) MIME-Version: 1.0 To: fortran standards email list for J3 Cc: Van.Snyder@jpl.nasa.gov, WG5 , MPI-3 Fortran working group Subject: Re: (j3.2006) (SC22WG5.3886) [ukfortran] [MPI3 Fortran] MPI non-blocking transfers References: <49776DF7.1040900@cray.com> <20090121211748.130A5C178D9@www2.open-std.org> <20090121224014.6CB63C178D9@www2.open-std.org> <20090121234200.4F3BDCA3434@www2.open-std.org> <20090122000407.D5A8ECA3434@www2.open-std.org> <20090122100652.C31E9CA3434@www2.open-std.org> In-Reply-To: <20090122100652.C31E9CA3434@www2.open-std.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 22 Jan 2009 17:08:01.0986 (UTC) FILETIME=[FBA82220:01C97CB3] X-Cray-VirusStatus: clean Sender: owner-sc22wg5@open-std.org Precedence: bulk N.M. Maclaren wrote: > On Jan 22 2009, Van Snyder wrote: > >> Assuming the problem is caused by the buffer not appearing as an actual >> argument, it's not a defect in Fortran but rather in the design of MPI. >> > > If that were the whole problem, it would be. It would also be a defect in > the design for the Fortran WAIT statement. > > Except that the WAIT statement does have at least some semantics that the compiler understands. Which raises an interesting point. Nick has pointed out several places where it is theoretically possible for the compiler optimizer to cause problems because the "invisible hand" of the MPI runtime currently "owns" part of the program's memory space and either expects it to remain static (sends) or modifies it (receives). Fortran already has an attribute designed exactly for this situation - VOLATILE. VOLATILE and ASYNCHRONOUS normally appear together in the standard since they are so similar in effect. The one difference (from the compiler's point of view) is that the volatility of a variable with the asynchronous attribute can be "turned off" when the compiler sees a global WAIT. If we modify the ASYNCHRONOUS attribute to be used to solve the issues with MPI calls, then WAIT is no longer special, and VOLATILE and ASYNCHRONOUS become functionally identical. That's one of the undesirable side effect that makes me dislike the idea of modifying ASYNCHRONOUS. (The other is that it incorrectly suggests that MPI calls have something to do with I/O which is not the case (except for the actual MPI I/O calls).) If we use a different name, like ASYNC_EXTERNAL, is there any difference between that and VOLATILE? I suspect not. In which case, the obvious solution is to just use VOLATILE. (Which was the conclusion of the previous round of discussions on this topic.) VOLATILE had a couple of very attractive features. It requires zero changes in either the Fortran standard or in any compiler implementations. Thus, it is an immediately available solution for the MPI group. VOLATILE might be strong enough to solve the code-motion problem as well. Cheers, Bill -- Bill Long longb@cray.com Fortran Technical Support & voice: 651-605-9024 Bioinformatics Software Development fax: 651-605-9142 Cray Inc., 1340 Mendota Heights Rd., Mendota Heights, MN, 55120