From owner-sc22wg5+sc22wg5-dom8=www.open-std.org@open-std.org Sat Mar 30 11:46:33 2013 Return-Path: X-Original-To: sc22wg5-dom8 Delivered-To: sc22wg5-dom8@www.open-std.org Received: by www.open-std.org (Postfix, from userid 521) id F05AA356DD0; Sat, 30 Mar 2013 11:46:32 +0100 (CET) Delivered-To: sc22wg5@open-std.org Received: from ppsw-41.csi.cam.ac.uk (ppsw-41.csi.cam.ac.uk [131.111.8.141]) by www.open-std.org (Postfix) with ESMTP id 77EEB356DC5 for ; Sat, 30 Mar 2013 11:46:31 +0100 (CET) X-Cam-AntiVirus: no malware found X-Cam-SpamDetails: not scanned X-Cam-ScannerInfo: http://www.ucs.cam.ac.uk/email/scanner/ Received: from hermes-1.csi.cam.ac.uk ([131.111.8.51]:55977) by ppsw-41.csi.cam.ac.uk (smtp.hermes.cam.ac.uk [131.111.8.156]:25) with esmtpa (EXTERNAL:nmm1) id 1ULtIp-0004Si-SV (Exim 4.72) (return-path ); Sat, 30 Mar 2013 10:46:27 +0000 Received: from prayer by hermes-1.csi.cam.ac.uk (hermes.cam.ac.uk) with local (PRAYER:nmm1) id 1ULtIp-0003YV-QN (Exim 4.72) (return-path ); Sat, 30 Mar 2013 10:46:27 +0000 Received: from [87.115.144.83] by webmail.hermes.cam.ac.uk with HTTP (Prayer-1.3.5); 30 Mar 2013 10:46:27 +0000 Date: 30 Mar 2013 10:46:27 +0000 From: "N.M. Maclaren" To: "Bader, Reinhold" Cc: "Van.Snyder@jpl.nasa.gov" , fortran standards email list for J3 , sc22wg5 Subject: Re: AW: (SC22WG5.4945) [ukfortran] AW: (j3.2006) Thoughts on Reinhold's thoughts Message-ID: In-Reply-To: <166ED263DF83324D9A3BA67FB6772B2B59F2B4D0@BADWLRZ-SWMBX11.ads.mwn.de> References: <20130329203623.0D66F356D96@www.open-std.org> <20130329212446.AD585356D97@www.open-std.org> <20130329231248.0A33D356DA7@www.open-std.org> <166ED263DF83324D9A3BA67FB6772B2B59F2B4D0@BADWLRZ-SWMBX11.ads.mwn.de> X-Mailer: Prayer v1.3.5 Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset=ISO-8859-1 Sender: owner-sc22wg5@open-std.org Precedence: bulk On Mar 30 2013, Bader, Reinhold wrote: > > I understand your concerns (and admit that I am also out of my depth > where the semantics of advanced memory models are concerned), but would > like to make the following comments: > > (1) The memory model papers you reference use a concept of computation > that does > not include synchronization operations. The discussion for Fortran > therefore needs > to focus on two different levels: The first one concerning > operations that permit > memory updates in unordered segments (atomics, events) - that's > where the memory model is needed - , and the second one that should > be allowed > to argue on the basis of *existing* synchronization semantics > (teams!). Yes and no. They do have synchronisation operations, actually, but they are semi-implicit. In C++, all accesses to atomic variables potentially imply a fence, and the default ones include a release/acquire fence. It's not a scalable model, and is almost unimplementable except on cache coherent shared memory. Also, the other aspect that makes me twitch is collectives. These provide a communication channel that does not involve any form of synchronisation. In that, they are very like atomics, but are not restricted to atomic variables :-( > (2) The concrete examples I'd like to see of course do not constitute > mathematical > proof. But they would provide valuable heuristics simple on whether > or not to > continue considering the second alternative. The trouble is that taking that approach has caused absolute chaos in the past, because this area is SO fiendish. People have been completely unable to produce examples that fail, programs have failed in real life, that has been tracked down to an inconsistency in the standard, and it has been then found to be unfixable :-( Both POSIX threads and OpenMP have that, redoubled in spades, and it has been 'resolved' in C and C++ only by incompatible changes to the languages. Even so, C++ has one serious inconsistency that I know of (which we blocked in Fortran), and C has dozens - even excluding the old ones of incompatibilities in the serial specification. In older languages, Modula 3 made a complete mess of this for the same reason, and there were almost certainly many others. If you can tell me a particular aspect that you are thinking of, I will see if I can create an example. But, currently, the proposals are just too inchoate (= just begun, with connotations of imperfect, undeveloped and immature) to do much more. Regards, Nick.