From owner-sc22wg5@open-std.org Thu Jun 11 10:41:45 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 1C09BC76BB7; Thu, 11 Jun 2009 10:41:45 +0200 (CET DST) X-Original-To: sc22wg5@open-std.org Delivered-To: sc22wg5@open-std.org Received: from ppsw-7.csi.cam.ac.uk (ppsw-7.csi.cam.ac.uk [131.111.8.137]) by www2.open-std.org (Postfix) with ESMTP id 671B7C4596B for ; Thu, 11 Jun 2009 10:41:20 +0200 (CET DST) X-Cam-AntiVirus: no malware found X-Cam-SpamDetails: not scanned X-Cam-ScannerInfo: http://www.cam.ac.uk/cs/email/scanner/ Received: from hermes-2.csi.cam.ac.uk ([131.111.8.54]:52501) by ppsw-7.csi.cam.ac.uk (smtp.hermes.cam.ac.uk [131.111.8.157]:25) with esmtpa (EXTERNAL:nmm1) id 1MEfqa-0006TW-NI (Exim 4.70) (return-path ); Thu, 11 Jun 2009 09:41:20 +0100 Received: from prayer by hermes-2.csi.cam.ac.uk (hermes.cam.ac.uk) with local (PRAYER:nmm1) id 1MEfqa-0003zq-6h (Exim 4.67) (return-path ); Thu, 11 Jun 2009 09:41:20 +0100 Received: from [83.67.89.123] by webmail.hermes.cam.ac.uk with HTTP (Prayer-1.3.1); 11 Jun 2009 09:41:20 +0100 Date: 11 Jun 2009 09:41:20 +0100 From: "N.M. Maclaren" To: WG5 Subject: Re: [ukfortran] (SC22WG5.4002) (j3.2006) New summary of coarrays Message-ID: In-Reply-To: <20090611020809.23E0FC4596B@www2.open-std.org> References: <20090609092023.32AF5C178DC@www2.open-std.org> <20090609093907.2863CC178DC@www2.open-std.org> <4A2E63B2.6020400@cray.com> <20090611020809.23E0FC4596B@www2.open-std.org> X-Mailer: Prayer v1.3.1 Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset=ISO-8859-1 Sender: owner-sc22wg5@open-std.org Precedence: bulk >Once again I reiterate my "watch what you are replying to" : J3 list >readers please use "Reply All" or manually reinsert the WG5 list when >replying to a WG5 message. Yes, please! I don't get J3 messages, for a start. >No, as I understand it existing hardware and physical constraints >encourages some vendors not to have global consistency; *and* we allow >them not to have global consistency. Yes. Bill is right that the chances of decent documentation are not good, unfortunately. I was hoping to encourage the vendors .... My reasoning for wanting to leave this open is that we really, but REALLY, can't predict where this area is going. There are two aspects: 1) Will Fortran coarrays be implement much on commodity clusters, what semantics will they deliver, how many programs will be written for them, and what semantics will they rely on? We haven't even selected the jury to answer that one yet! 2) What memory model will the multi-core CPUs of 2019 (128 is estimated) use? Few (none?) of the current ones deliver sequential consistency, and some experts believe that even causal consistency will have to be abandoned to get decent efficiency on those systems. And that is why I believe that tying down the memory model should be a job for Fortran 2018. The current wording is clean (and clear to an expert), and does not prejudge a future decision. >As for "confusing to an ordinary user", not nearly as confusing as >getting wildly different answers or program hangs/crashes from the same >data on the same machine on a Thursday. Precisely - been there, seen that, spent hours trying to explain it to the users :-( That is why I feel that there should be a warning in all relevant documents "Here Be Dragons", but I don't care how it is phrased. Regards, Nick Maclaren, University of Cambridge Computing Service, New Museums Site, Pembroke Street, Cambridge CB2 3QH, England. Email: nmm1@cam.ac.uk Tel.: +44 1223 334761 Fax: +44 1223 334679