From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: [RFC][PATCH v3] readahead: introduce O_RANDOM for POSIX_FADV_RANDOM Date: Mon, 4 Jan 2010 12:50:20 +0800 Message-ID: <20100104045020.GA21021@localhost> References: <20091225000717.GA26949@yahoo-inc.com> <87aax18xms.fsf@basil.nowhere.org> <20091230051540.GA16308@localhost> <20091230052402.GB26364@localhost> <873a2s8hmp.fsf@basil.nowhere.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andrew Morton , Quentin Barnes , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , Nick Piggin , Steven Whitehouse , Andi Kleen , David Howells , Al Viro , Jonathan Corbet , Christoph Hellwig To: Andi Kleen Return-path: Received: from mga03.intel.com ([143.182.124.21]:50159 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750738Ab0ADFFx (ORCPT ); Mon, 4 Jan 2010 00:05:53 -0500 Content-Disposition: inline In-Reply-To: <873a2s8hmp.fsf@basil.nowhere.org> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: This fixes inefficient page-by-page reads on POSIX_FADV_RANDOM. POSIX_FADV_RANDOM used to set ra_pages=0, which leads to poor performance: a 16K read will be carried out in 4 _sync_ 1-page reads. In other places, ra_pages==0 means - it's ramfs/tmpfs/hugetlbfs/sysfs/configfs - some IO error happened where multi-page read IO won't help or should be avoided. POSIX_FADV_RANDOM actually want a different semantics: to disable the *heuristic* readahead algorithm, and to use a dumb one which faithfully submit read IO for whatever application requests. So introduce a flag O_RANDOM for POSIX_FADV_RANDOM. It will be visible to fcntl(F_GETFL). Note that the random hint is not likely to help random reads performance noticeably. And it may be too permissive on huge request size (its IO size is not limited by read_ahead_kb). In Quentin's report (http://lkml.org/lkml/2009/12/24/145), the overall (NFS read) performance of the application increased by 313%! v3: use O_RANDOM to indicate both read/write access pattern as in posix_fadvise(), although it only takes effect for read() now (proposed by Quentin) v2: use O_RANDOM_READ to avoid race conditions (pointed out by Andi) CC: Nick Piggin CC: Andi Kleen CC: Steven Whitehouse CC: David Howells CC: Al Viro CC: Jonathan Corbet CC: Christoph Hellwig Tested-by: Quentin Barnes Signed-off-by: Wu Fengguang --- include/asm-generic/fcntl.h | 4 ++++ mm/fadvise.c | 10 +++++++++- mm/readahead.c | 6 ++++++ 3 files changed, 19 insertions(+), 1 deletion(-) --- linux.orig/include/asm-generic/fcntl.h 2010-01-04 12:39:29.000000000 +0800 +++ linux/include/asm-generic/fcntl.h 2010-01-04 12:40:11.000000000 +0800 @@ -80,6 +80,10 @@ #define O_NDELAY O_NONBLOCK #endif +#ifndef O_RANDOM +#define O_RANDOM 010000000 /* random access pattern hint */ +#endif + #define F_DUPFD 0 /* dup */ #define F_GETFD 1 /* get close_on_exec */ #define F_SETFD 2 /* set/clear close_on_exec */ --- linux.orig/mm/fadvise.c 2010-01-04 12:39:29.000000000 +0800 +++ linux/mm/fadvise.c 2010-01-04 12:39:30.000000000 +0800 @@ -77,12 +77,20 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, lof switch (advice) { case POSIX_FADV_NORMAL: file->f_ra.ra_pages = bdi->ra_pages; + spin_lock(&file->f_lock); + file->f_flags &= ~O_RANDOM; + spin_unlock(&file->f_lock); break; case POSIX_FADV_RANDOM: - file->f_ra.ra_pages = 0; + spin_lock(&file->f_lock); + file->f_flags |= O_RANDOM; + spin_unlock(&file->f_lock); break; case POSIX_FADV_SEQUENTIAL: file->f_ra.ra_pages = bdi->ra_pages * 2; + spin_lock(&file->f_lock); + file->f_flags &= ~O_RANDOM; + spin_unlock(&file->f_lock); break; case POSIX_FADV_WILLNEED: if (!mapping->a_ops->readpage) { --- linux.orig/mm/readahead.c 2010-01-04 12:39:29.000000000 +0800 +++ linux/mm/readahead.c 2010-01-04 12:39:30.000000000 +0800 @@ -501,6 +501,12 @@ void page_cache_sync_readahead(struct ad if (!ra->ra_pages) return; + /* be dumb */ + if (filp->f_flags & O_RANDOM) { + force_page_cache_readahead(mapping, filp, offset, req_size); + return; + } + /* do read-ahead */ ondemand_readahead(mapping, ra, filp, false, offset, req_size); }