From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757014Ab2JZHrg (ORCPT ); Fri, 26 Oct 2012 03:47:36 -0400 Received: from mail-ob0-f174.google.com ([209.85.214.174]:57606 "EHLO mail-ob0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756985Ab2JZHrc (ORCPT ); Fri, 26 Oct 2012 03:47:32 -0400 Message-ID: <508A4007.5080906@gmail.com> Date: Fri, 26 Oct 2012 15:47:19 +0800 From: Ni zhan Chen User-Agent: Mozilla/5.0 (X11; Linux i686; rv:16.0) Gecko/20121011 Thunderbird/16.0.1 MIME-Version: 1.0 To: Fengguang Wu CC: Dave Chinner , YingHang Zhu , akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state References: <50889FF1.9030107@gmail.com> <20121025025826.GB23462@localhost> <20121026002544.GI29378@dastard> <20121026012758.GA6282@localhost> <5089F5AD.5040708@gmail.com> <20121026065855.GA9179@localhost> <508A35B0.30106@gmail.com> <20121026070936.GA12282@localhost> <508A399D.6000506@gmail.com> <20121026073630.GA12886@localhost> In-Reply-To: <20121026073630.GA12886@localhost> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/26/2012 03:36 PM, Fengguang Wu wrote: > On Fri, Oct 26, 2012 at 03:19:57PM +0800, Ni zhan Chen wrote: >> On 10/26/2012 03:09 PM, Fengguang Wu wrote: >>> On Fri, Oct 26, 2012 at 03:03:12PM +0800, Ni zhan Chen wrote: >>>> On 10/26/2012 02:58 PM, Fengguang Wu wrote: >>>>>> static void shrink_readahead_size_eio(struct file *filp, >>>>>> struct file_ra_state *ra) >>>>>> { >>>>>> - ra->ra_pages /= 4; >>>>>> + spin_lock(&filp->f_lock); >>>>>> + filp->f_mode |= FMODE_RANDOM; >>>>>> + spin_unlock(&filp->f_lock); >>>>>> >>>>>> As the example in comment above this function, the read maybe still >>>>>> sequential, and it will waste IO bandwith if modify to FMODE_RANDOM >>>>>> directly. >>>>> Yes immediately disabling readahead may hurt IO performance, the >>>>> original '/ 4' may perform better when there are only 1-3 IO errors >>>>> encountered. >>>> Hi Fengguang, >>>> >>>> Why the number should be 1-3? >>> The original behavior is '/= 4' on each error. >>> >>> After 1 errors, readahead size will be shrinked by 1/4 >>> After 2 errors, readahead size will be shrinked by 1/16 >>> After 3 errors, readahead size will be shrinked by 1/64 >>> After 4 errors, readahead size will be effectively 0 (disabled) >> But from function shrink_readahead_size_eio and its caller >> filemap_fault I can't find the behavior you mentioned. How you >> figure out it? > It's this line in shrink_readahead_size_eio(): > > ra->ra_pages /= 4; Yeah, I mean why the 4th readahead size will be 0(disabled)? What's the original value of ra->ra_pages? How can guarantee the 4th shrink readahead size can be 0? Regards, Chen > > That ra_pages will keep shrinking by 4 on each error. The only way to > restore it is to reopen the file, or POSIX_FADV_SEQUENTIAL. > > Thanks, > Fengguang >