From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261419AbULXSVw (ORCPT ); Fri, 24 Dec 2004 13:21:52 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261420AbULXSVm (ORCPT ); Fri, 24 Dec 2004 13:21:42 -0500 Received: from fw.osdl.org ([65.172.181.6]:35025 "EHLO mail.osdl.org") by vger.kernel.org with ESMTP id S261413AbULXSVg (ORCPT ); Fri, 24 Dec 2004 13:21:36 -0500 Date: Fri, 24 Dec 2004 10:21:24 -0800 (PST) From: Linus Torvalds To: Arjan van de Ven cc: Paul Mackerras , Christoph Lameter , Andrew Morton , linux-ia64@vger.kernel.org, linux-mm@kvack.org, Kernel Mailing List Subject: Re: Prezeroing V2 [0/3]: Why and When it works In-Reply-To: <1103879668.4131.15.camel@laptopd505.fenrus.org> Message-ID: References: <41C20E3E.3070209@yahoo.com.au> <16843.13418.630413.64809@cargo.ozlabs.ibm.com> <1103879668.4131.15.camel@laptopd505.fenrus.org> 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 On Fri, 24 Dec 2004, Arjan van de Ven wrote: > > problem is.. will it buy you anything if you use the page again > anyway... since such pages will be cold cached now. So for sure some of > it is only shifting latency from kernel side to userspace side, but > readprofile doesn't measure the later so it *looks* better... Absolutely. I would want to see some real benchmarks before we do this. Not just some microbenchmark of "how many page faults can we take without _using_ the page at all". I agree 100% with you that we shouldn't shift the costs around. Having a hice hot-spot that we know about is a good thing, and it means that performance profiles show what the time is really spent on. Often getting rid of the hotspot just smears out the work over a wider area, making other optimizations (like trying to make the memory footprint _smaller_ and removing the work entirely that way) totally impossible because now the performance profile just has a constant background noise and you can't tell what the real problem is. Linus