From mboxrd@z Thu Jan 1 00:00:00 1970 From: Goswin von Brederlow Subject: Re: [00/41] Large Blocksize Support V7 (adds memmap support) Date: Sat, 15 Sep 2007 14:14:42 +0200 Message-ID: <87ir6c3z2l.fsf@informatik.uni-tuebingen.de> References: <20070911060349.993975297@sgi.com> <200709110452.20363.nickpiggin@yahoo.com.au> <20070911121225.GE13132@lazybastard.org> <20070915014449.4f9cdb51.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Joern Engel , Nick Piggin , Christoph Lameter , andrea@suse.de, torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , Mel Gorman , William Lee Irwin III , David Chinner , Jens Axboe , Badari Pulavarty , Maxim Levitsky , Fengguang Wu , swin wang , totty.lu@gmail.com, hugh@veritas.com To: Andrew Morton Return-path: Received: from mx1.Informatik.Uni-Tuebingen.De ([134.2.12.5]:37787 "EHLO mx1.informatik.uni-tuebingen.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750945AbXIOMPK (ORCPT ); Sat, 15 Sep 2007 08:15:10 -0400 In-Reply-To: <20070915014449.4f9cdb51.akpm@linux-foundation.org> (Andrew Morton's message of "Sat, 15 Sep 2007 01:44:49 -0700") Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Andrew Morton writes: > On Tue, 11 Sep 2007 14:12:26 +0200 J=F6rn Engel wro= te: > >> While I agree with your concern, those numbers are quite silly. The >> chances of 99.8% of pages being free and the remaining 0.2% being >> perfectly spread across all 2MB large_pages are lower than those of = SHA1 >> creating a collision. > > Actually it'd be pretty easy to craft an application which allocates = seven > pages for pagecache, then one for , then seven for pagecac= he, then > one for , etc. > > I've had test apps which do that sort of thing accidentally. The res= ult > wasn't pretty. Except that the applications 7 pages are movable and the would have to be unmovable. And then they should not share the same memory region. At least they should never be allowed to interleave in such a pattern on a larger scale. The only way a fragmentation catastroph can be (proovable) avoided is by having so few unmovable objects that size + max waste << ram size. The smaller the better. Allowing movable and unmovable objects to mix means that max waste goes way up. In your example waste would be 7*size. With 2MB uper order limit it would be 511*size. I keep coming back to the fact that movable objects should be moved out of the way for unmovable ones. Anything else just allows fragmentation to build up. MfG Goswin - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel= " in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html