Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752626AbXBJBja (ORCPT ); Fri, 9 Feb 2007 20:39:30 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752617AbXBJBja (ORCPT ); Fri, 9 Feb 2007 20:39:30 -0500 Received: from smtp.ustc.edu.cn ([202.38.64.16]:51266 "HELO ustc.edu.cn" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with SMTP id S1752626AbXBJBj3 (ORCPT ); Fri, 9 Feb 2007 20:39:29 -0500 Message-ID: <371071566.30389@ustc.edu.cn> X-EYOUMAIL-SMTPAUTH: wfg@mail.ustc.edu.cn Date: Sat, 10 Feb 2007 09:40:49 +0800 From: Fengguang Wu To: Andrew Morton Cc: Suparna Bhattacharya , John Tran , Mike Sullivan , Chris Mason , Steven Pratt , Ram Pai , Badari Pulavarty , Mingming Cao , Linux Kernel Subject: [PATCH -mm] readahead: partial sendfile fix Message-ID: <20070210014049.GA11269@mail.ustc.edu.cn> Mail-Followup-To: Andrew Morton , Suparna Bhattacharya , John Tran , Mike Sullivan , Chris Mason , Steven Pratt , Ram Pai , Badari Pulavarty , Mingming Cao , Linux Kernel MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2842 Lines: 76 Enable readahead to handle partially done read requests, e.g. sendfile(188, 1921, [1478592], 19553028) = 37440 sendfile(188, 1921, [1516032], 19515588) = 28800 sendfile(188, 1921, [1544832], 19486788) = 37440 sendfile(188, 1921, [1582272], 19449348) = 14400 sendfile(188, 1921, [1596672], 19434948) = 37440 sendfile(188, 1921, [1634112], 19397508) = 37440 In the above strace log, - some lighttpd is doing _sequential_ reading - every sendfile() returns with only _partial_ work done page_cache_readahead() expects that if it returns @next_index, it will be called exactly at @next_index next time. That's not true here. So the pattern will be falsely recognized as a random read trace. Also documented in "Linux AIO Performance and Robustness for Enterprise Workloads" section 3.5: sendfile(fd, 0, 2GB, fd2) = 8192, tells readahead about up to 128KB of the read sendfile(fd, 8192, 2GB - 8192, fd2) = 8192, tells readahead about 8KB - 132KB of the read sendfile(fd, 16384, 2GB - 16384, fd2) = 8192, tells readahead about 16KB-140KB of the read ... This confuses the readahead logic about the I/O pattern which appears to be 0-128K, 8K-132K, 16K-140K instead of clear sequentiality from 0-2GB that is really appropriate. Retry based AIO shares the same read pattern and readahead problem. In this case, simply disabling readahead on restarted aio is not a good option: we still need to call into readahead in the rare case of (req_size > ra_max). Signed-off-by: Fengguang Wu --- mm/filemap.c | 3 --- mm/readahead.c | 9 +++++++++ 2 files changed, 9 insertions(+), 3 deletions(-) --- linux-2.6.20-rc6-mm3.orig/mm/readahead.c +++ linux-2.6.20-rc6-mm3/mm/readahead.c @@ -581,6 +581,15 @@ page_cache_readahead(struct address_spac int sequential; /* + * A previous read request is partially completed, + * causing the retried/continued read calls into us prematurely. + */ + if (ra->start < offset && + offset < ra->prev_page && + ra->prev_page < ra->ahead_start + ra->ahead_size) + goto out; + + /* * We avoid doing extra work and bogusly perturbing the readahead * window expansion logic. */ --- linux-2.6.20-rc6-mm3.orig/mm/filemap.c +++ linux-2.6.20-rc6-mm3/mm/filemap.c @@ -915,9 +915,6 @@ void do_generic_mapping_read(struct addr if (!isize) goto out; - if (unlikely(aio_restarted())) - next_index = last_index; /* Avoid repeat readahead */ - end_index = (isize - 1) >> PAGE_CACHE_SHIFT; for (;;) { struct page *page; - 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/