Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753673Ab2JZCaU (ORCPT ); Thu, 25 Oct 2012 22:30:20 -0400 Received: from mail-ia0-f174.google.com ([209.85.210.174]:40427 "EHLO mail-ia0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752737Ab2JZCaT (ORCPT ); Thu, 25 Oct 2012 22:30:19 -0400 Message-ID: <5089F5AD.5040708@gmail.com> Date: Fri, 26 Oct 2012 10:30:05 +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: <1350996411-5425-1-git-send-email-casualfisher@gmail.com> <20121023224706.GR4291@dastard> <20121024201921.GX4291@dastard> <20121025015014.GC29378@dastard> <50889FF1.9030107@gmail.com> <20121025025826.GB23462@localhost> <20121026002544.GI29378@dastard> <20121026012758.GA6282@localhost> In-Reply-To: <20121026012758.GA6282@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 Content-Length: 2701 Lines: 64 On 10/26/2012 09:27 AM, Fengguang Wu wrote: > On Fri, Oct 26, 2012 at 11:25:44AM +1100, Dave Chinner wrote: >> On Thu, Oct 25, 2012 at 10:58:26AM +0800, Fengguang Wu wrote: >>> Hi Chen, >>> >>>> But how can bdi related ra_pages reflect different files' readahead >>>> window? Maybe these different files are sequential read, random read >>>> and so on. >>> It's simple: sequential reads will get ra_pages readahead size while >>> random reads will not get readahead at all. >>> >>> Talking about the below chunk, it might hurt someone that explicitly >>> takes advantage of the behavior, however the ra_pages*2 seems more >>> like a hack than general solution to me: if the user will need >>> POSIX_FADV_SEQUENTIAL to double the max readahead window size for >>> improving IO performance, then why not just increase bdi->ra_pages and >>> benefit all reads? One may argue that it offers some differential >>> behavior to specific applications, however it may also present as a >>> counter-optimization: if the root already tuned bdi->ra_pages to the >>> optimal size, the doubled readahead size will only cost more memory >>> and perhaps IO latency. >>> >>> --- a/mm/fadvise.c >>> +++ b/mm/fadvise.c >>> @@ -87,7 +86,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice) >>> spin_unlock(&file->f_lock); >>> break; >>> case POSIX_FADV_SEQUENTIAL: >>> - file->f_ra.ra_pages = bdi->ra_pages * 2; >> I think we really have to reset file->f_ra.ra_pages here as it is >> not a set-and-forget value. e.g. shrink_readahead_size_eio() can >> reduce ra_pages as a result of IO errors. Hence if you have had io >> errors, telling the kernel that you are now going to do sequential >> IO should reset the readahead to the maximum ra_pages value >> supported.... > Good point! > > .... but wait .... this patch removes file->f_ra.ra_pages in all other > places too, so there will be no file->f_ra.ra_pages to be reset here... In his patch, 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. > > Thanks, > Fengguang > -- 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/