Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762264AbXF0Spo (ORCPT ); Wed, 27 Jun 2007 14:45:44 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755946AbXF0Spg (ORCPT ); Wed, 27 Jun 2007 14:45:36 -0400 Received: from x35.xmailserver.org ([64.71.152.41]:1035 "EHLO x35.xmailserver.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753806AbXF0Spg (ORCPT ); Wed, 27 Jun 2007 14:45:36 -0400 X-AuthUser: davidel@xmailserver.org Date: Wed, 27 Jun 2007 11:45:33 -0700 (PDT) From: Davide Libenzi X-X-Sender: davide@alien.or.mcafeemobile.com To: Hugh Dickins 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: X-GPG-FINGRPRINT: CFAE 5BEE FD36 F65E E640 56FE 0974 BF23 270F 474E X-GPG-PUBLIC_KEY: http://www.xmailserver.org/davidel.asc MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1917 Lines: 43 On Wed, 27 Jun 2007, Hugh Dickins wrote: > 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. The reason why I posted is exactly so other ppl can look at it and find possible flaws in the way pages and retired. If an effective UID was able to see (or it generated) the data on that page, it should be able to get that page back uncleared (when VM_NOZERO is set). >From a performance POV, a 2-3% boost on a non-micro-bench test like a kernel build is not exaclty peanuts. And for more heavy malloc/anon-mmap appliations, the boost goes up to 10-15%. That is not exactly what I call "little time" ;) - Davide - 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/