From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx193.postini.com [74.125.245.193]) by kanga.kvack.org (Postfix) with SMTP id 59BCD6B0072 for ; Thu, 25 Oct 2012 22:25:09 -0400 (EDT) Received: by mail-oa0-f41.google.com with SMTP id k14so2845094oag.14 for ; Thu, 25 Oct 2012 19:25:08 -0700 (PDT) Message-ID: <5089F47E.5010405@gmail.com> Date: Fri, 26 Oct 2012 10:25:02 +0800 From: Ni zhan Chen MIME-Version: 1.0 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> In-Reply-To: <20121026002544.GI29378@dastard> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Dave Chinner Cc: Fengguang Wu , YingHang Zhu , akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org On 10/26/2012 08:25 AM, 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 catch! > > Cheers, > > Dave. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org