Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934113Ab3HHIyi (ORCPT ); Thu, 8 Aug 2013 04:54:38 -0400 Received: from mga11.intel.com ([192.55.52.93]:1381 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933987Ab3HHIyg (ORCPT ); Thu, 8 Aug 2013 04:54:36 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.89,838,1367996400"; d="scan'208";a="383148229" Date: Thu, 8 Aug 2013 16:54:18 +0800 From: Fengguang Wu To: Andrew Morton Cc: Miao Xie , Tao Ma , Linux Memory Management List , LKML Subject: readahead: make context readahead more conservative Message-ID: <20130808085418.GA23970@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2810 Lines: 80 This helps performance on moderately dense random reads on SSD. Transaction-Per-Second numbers provided by Taobao: QPS case ------------------------------------------------------- 7536 disable context readahead totally w/ patch: 7129 slower size rampup and start RA on the 3rd read 6717 slower size rampup w/o patch: 5581 unmodified context readahead Before, readahead will be started whenever reading page N+1 when it happen to read N recently. After patch, we'll only start readahead when *three* random reads happen to access pages N, N+1, N+2. The probability of this happening is extremely low for pure random reads, unless they are very dense, which actually deserves some readahead. Also start with a smaller readahead window. The impact to interleaved sequential reads should be small, because for a long run stream, the the small readahead window rampup phase is negletable. The context readahead actually benefits clustered random reads on HDD whose seek cost is pretty high. However as SSD is increasingly used for random read workloads it's better for the context readahead to concentrate on interleaved sequential reads. Another SSD rand read test from Miao # file size: 2GB # read IO amount: 625MB sysbench --test=fileio \ --max-requests=10000 \ --num-threads=1 \ --file-num=1 \ --file-block-size=64K \ --file-test-mode=rndrd \ --file-fsync-freq=0 \ --file-fsync-end=off run shows the performance of btrfs grows up from 69MB/s to 121MB/s, ext4 from 104MB/s to 121MB/s. Tested-by: Tao Ma Tested-by: Miao Xie Signed-off-by: Wu Fengguang --- mm/readahead.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) --- linux-next.orig/mm/readahead.c 2013-08-08 16:21:29.675286154 +0800 +++ linux-next/mm/readahead.c 2013-08-08 16:21:33.851286019 +0800 @@ -371,10 +371,10 @@ static int try_context_readahead(struct size = count_history_pages(mapping, ra, offset, max); /* - * no history pages: + * not enough history pages: * it could be a random read */ - if (!size) + if (size <= req_size) return 0; /* @@ -385,8 +385,8 @@ static int try_context_readahead(struct size *= 2; ra->start = offset; - ra->size = get_init_ra_size(size + req_size, max); - ra->async_size = ra->size; + ra->size = min(size + req_size, max); + ra->async_size = 1; return 1; } -- 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/