Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760747AbZJIMVa (ORCPT ); Fri, 9 Oct 2009 08:21:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760636AbZJIMVa (ORCPT ); Fri, 9 Oct 2009 08:21:30 -0400 Received: from casper.infradead.org ([85.118.1.10]:43959 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760615AbZJIMV3 (ORCPT ); Fri, 9 Oct 2009 08:21:29 -0400 Subject: Re: [PATCH] mm: make VM_MAX_READAHEAD configurable From: Peter Zijlstra To: Ehrhardt Christian Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jens Axboe , Andrew Morton , Martin Schwidefsky , Wu Fengguang In-Reply-To: <1255087175-21200-1-git-send-email-ehrhardt@linux.vnet.ibm.com> References: <1255087175-21200-1-git-send-email-ehrhardt@linux.vnet.ibm.com> Content-Type: text/plain Date: Fri, 09 Oct 2009 14:20:30 +0200 Message-Id: <1255090830.8802.60.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1429 Lines: 30 On Fri, 2009-10-09 at 13:19 +0200, Ehrhardt Christian wrote: > From: Christian Ehrhardt > > On one hand the define VM_MAX_READAHEAD in include/linux/mm.h is just a default > and can be configured per block device queue. > On the other hand a lot of admins do not use it, therefore it is reasonable to > set a wise default. > > This path allows to configure the value via Kconfig mechanisms and therefore > allow the assignment of different defaults dependent on other Kconfig symbols. > > Using this, the patch increases the default max readahead for s390 improving > sequential throughput in a lot of scenarios with almost no drawbacks (only > theoretical workloads with a lot concurrent sequential read patterns on a very > low memory system suffer due to page cache trashing as expected). Why can't this be solved in userspace? Also, can't we simply raise this number if appropriate? Wu did some read-ahead trashing detection bits a long while back which should scale the read-ahead window back when we're low on memory, not sure that ever made it in, but that sounds like a better option than having different magic numbers for each platform. -- 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/