From owner-sc22wg5@open-std.org Thu Nov 6 21:07:42 2008 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 565BCCA5FE8; Thu, 6 Nov 2008 21:07:42 +0100 (CET) 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 C771DCA343A for ; Thu, 6 Nov 2008 21:07:40 +0100 (CET) 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-1.csi.cam.ac.uk ([131.111.8.51]:60236) by ppsw-7.csi.cam.ac.uk (smtp.hermes.cam.ac.uk [131.111.8.157]:25) with esmtpa (EXTERNAL:nmm1) id 1KyB8m-0003kN-Oc (Exim 4.70) for sc22wg5@open-std.org (return-path ); Thu, 06 Nov 2008 20:07:40 +0000 Received: from prayer by hermes-1.csi.cam.ac.uk (hermes.cam.ac.uk) with local (PRAYER:nmm1) id 1KyB8m-0001gi-Jz (Exim 4.67) for sc22wg5@open-std.org (return-path ); Thu, 06 Nov 2008 20:07:40 +0000 Received: from [83.67.89.123] by webmail.hermes.cam.ac.uk with HTTP (Prayer-1.3.1); 06 Nov 2008 20:07:40 +0000 Date: 06 Nov 2008 20:07:40 +0000 From: "N.M. Maclaren" To: sc22wg5@open-std.org Subject: Re: [ukfortran] (SC22WG5.3634) (j3.2006) N1751 Message-ID: In-Reply-To: <20081106184403.8DF6ACA343A@www2.open-std.org> References: <20081106184403.8DF6ACA343A@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 On Nov 6 2008, Michael Ingrassia wrote: > >>Can you point me at anything in the standard that states that principle? > > 2.4.2p1 says that Execution of a program consists of the asynchronous > execution of a fixed number of images. Chapter 8 describes the control > flow within an image. Failure to execute image P is a failure to execute > the program. Progress in image P can be paused at PAUSE statements or at > image control statements, but as you say I think that failure to advance > otherwise might be considered "gratuitously perverse". Yes, but the problem is that the program IS "advancing" as fast as it can! What the processor can't tell is that it is merely running on the spot. Note that I am not saying that Fortran should specify fairness in scheduling (no other language does that I know of), but that NOTE 8.38 relies on it, and it is therefore undefined whether that code will ever complete. >Yes, anyone can write non-halting programs and a processor can't weed >them out a priori. That doesn't mean that users must always despair; by >use of discipline they can write programs which they can prove >will in fact halt. I've seen lots of such proofs. Oh, yes, I could give you a delightful probabilistic example that I gave to someone talking on this in 1972 :-) My point is that you CAN'T do it with current VOLATILE coarrays, without specifying 'fairness'. > The question is whether the (implied) proof that this program in NOTE > 8.38 halts can be supplied based on our standard or whether it requires > extra assumptions. I think we have to assume that the system scheduler > respects the Fortran concept of image control statements. My claim is > that a scheduler consistent with the execution rules in the standard > can't permanently swap out an image which is not suspended at an image > control statement. Or so I thought. Maybe that means existing thread > schedulers would need to learn a wee bit of Fortran to become image > schedulers ? That is a vain hope. Many people have been trying to get general-purpose schedulers to support gang-scheduling and similar 'HPC' models for decades, with virtually no progress. >BTW your mail headers included > >>To: sc22wg5@open-std.org >>Reply-to: fortran standards email list for J3 > >which seems odd to me. I don't understand why replies aren't directed >to sc22wg5. Bloody Webmail. I have been inflicted with a new system for mail, and it is . Sorry. 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