From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753289AbZL3UbI (ORCPT ); Wed, 30 Dec 2009 15:31:08 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752393AbZL3UbH (ORCPT ); Wed, 30 Dec 2009 15:31:07 -0500 Received: from mail-yw0-f176.google.com ([209.85.211.176]:53897 "EHLO mail-yw0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752238AbZL3UbG convert rfc822-to-8bit (ORCPT ); Wed, 30 Dec 2009 15:31:06 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=Am2H7XFZkG+2eljWJ1EJk2n5HO1mL+Ay29cTKXPT8uTg3fIGyspzsMS+oC5SY/kmC2 VgDq2XDD/hmqniB6jPvlhYwx2zWZMPjtzuDV3G+tLypW9uH4VpYlDTnkDQxLLQp03FJB R8r2NmK5RfyqguzwsVzpTyvlnlHyuvB6GSf/A= MIME-Version: 1.0 In-Reply-To: <20091230184535.GE4489@kernel.dk> References: <1262175004-2132-1-git-send-email-czoccolo@gmail.com> <20091230184535.GE4489@kernel.dk> Date: Wed, 30 Dec 2009 21:31:03 +0100 Message-ID: <4e5e476b0912301231k3214bb3dicbfc59d94623d72@mail.gmail.com> Subject: Re: [PATCH] cfq-iosched: non-rot devices do not need queue merging From: Corrado Zoccolo To: Jens Axboe Cc: Linux-Kernel , Jeff Moyer , Vivek Goyal , Shaohua Li , Gui Jianfeng Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 30, 2009 at 7:45 PM, Jens Axboe wrote: > On Wed, Dec 30 2009, Corrado Zoccolo wrote: >> Non rotational devices' performances are not affected by >> distance of requests, so there is no point in having overhead >> to merge queues of nearby requests. > > If the distance is zero, it may still make a big difference (at least > for writes). This check would be better as "ncq and doesn't suck", ala > >        blk_queue_nonrot(q) && tagged > > like we do elsewhere. For reads, though, even flash cards and netbook ssds are completely unaffected. I have done few experiments on my available disks: * http://dl.dropbox.com/u/3525644/service_time.png (I used the program: http://dl.dropbox.com/u/3525644/stride.c to get the graphs). For distance 0, I think request merging will be more effective than queue merging, moreover I think the multi-thread trick to have large I/O depth is used for reads, not writes (where simply issuing buffered writes already achieves a similar effect), so I think it is safe to disable it for all non-rotational devices. Thanks, Corrado > > -- > Jens Axboe > >