From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750944AbWFZU0v (ORCPT ); Mon, 26 Jun 2006 16:26:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750870AbWFZU0v (ORCPT ); Mon, 26 Jun 2006 16:26:51 -0400 Received: from mx1.suse.de ([195.135.220.2]:5014 "EHLO mx1.suse.de") by vger.kernel.org with ESMTP id S1750732AbWFZU0u (ORCPT ); Mon, 26 Jun 2006 16:26:50 -0400 To: Linus Torvalds Cc: Linux Kernel Mailing List , akpm@osdl.org, hyoshiok@miraclelinux.com Subject: Re: [PATCH] x86: cache pollution aware __copy_from_user_ll() References: <200606231501.k5NF1B79002899@hera.kernel.org> <1151160152.3181.59.camel@laptopd505.fenrus.org> From: Andi Kleen Date: 26 Jun 2006 22:26:47 +0200 In-Reply-To: Message-ID: User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3 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 Linus Torvalds writes: > On Sat, 24 Jun 2006, Arjan van de Ven wrote: > > > > while this patch will reduce the number of cycles spent in the kernel, > > it's just pushing the cache miss to userspace (by virtue of doing a > > cache flush effectively)... is this really the right thing? The total > > memory bandwidth will actually increase with this patch if you're > > unlucky (eg if userspace decides to write to this memory eventually).... > > No. It's for copying _from_ user space, ie a "write()" system call. So > what it does is to effectively try to use non-temporal stores to the page > cache - since the page cache is usually not read directly afterwards (at > least not soon enough for L1 caches to help). > > I don't generally like cache tricks either (caches tend to be better than > humans, or at least get there fairly soon), but this one does seem very > valid. Problem is that it will likely wreck pipe and AF_UNIX (= X server) performance. These read the buffer quickly again. If you do something like this it would be better to define a special function and use it for real FS traffic only. -Andi