Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761650AbXF0Sdb (ORCPT ); Wed, 27 Jun 2007 14:33:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757259AbXF0SdX (ORCPT ); Wed, 27 Jun 2007 14:33:23 -0400 Received: from extu-mxob-1.symantec.com ([216.10.194.28]:48828 "EHLO extu-mxob-1.symantec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752546AbXF0SdX (ORCPT ); Wed, 27 Jun 2007 14:33:23 -0400 Date: Wed, 27 Jun 2007 19:32:47 +0100 (BST) From: Hugh Dickins X-X-Sender: hugh@blonde.wat.veritas.com To: Davide Libenzi cc: Ulrich Drepper , blaisorblade@yahoo.it, Linux Kernel Mailing List Subject: Re: [patch 2/3] MAP_NOZERO - implement sys_brk2() In-Reply-To: Message-ID: References: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Brightmail-Verdict: VlJEQwAAAAIAAAABAAAAAAAAAAEAAAAAAAAABGluYm94AGJsYWlzb3JibGFkZUB5YWhvby5pdABkYXZpZGVsQHhtYWlsc2VydmVyLm9yZwBsaW51eC1rZXJuZWxAdmdlci5rZXJuZWwub3JnAGRyZXBwZXJAZ21haWwuY29tAA== X-Brightmail-Tracker: AAAAAA== Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1633 Lines: 35 On Wed, 27 Jun 2007, Davide Libenzi wrote: > On Wed, 27 Jun 2007, Hugh Dickins wrote: > > > In honesty, I should add that I dislike and distrust Davide's > > MAP_NOZERO very much indeed! Would much rather leave my cpus > > spending a little time in clear_page(). A uid in struct page > > (though I'm sure we could find somewhere to tuck it away) - > > the horror, the horror! But I've so far failed to find a killer > > argument against it, and am hoping for someone else to do so. > > Little time? Please, do not trust me. Start oprofile and run a kernel > build. Look, I'm not even talking about som micro benchmark explicitly > built to exploit the thing. A kernel build. > You will find clear_page to be the *1st* kernel entry after cc1 and as. > That is bad for two reasons. The time it spends in there, and the cache it > blows. I don't doubt that it shows real benefits; but dangerously cutting corners usually shows benefits too. Relying on a uid at this level feels very wrong to me - but as I said, I've not found a killer argument against it. And we both know that clear_page features so high in part because it's bringing cachelines in from the cold, which are about to be accessed again by userspace; so it's often not so bad as it appears. Though I probably wouldn't be citing that argument if we were talking about offloading clear_page to another engine. Hugh - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/