From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754512Ab2A0Dkk (ORCPT ); Thu, 26 Jan 2012 22:40:40 -0500 Received: from mga14.intel.com ([143.182.124.37]:41153 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751875Ab2A0Dkh (ORCPT ); Thu, 26 Jan 2012 22:40:37 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.71,315,1320652800"; d="scan'208";a="100232689" Message-Id: <20120127031326.469063803@intel.com> User-Agent: quilt/0.48-1 Date: Fri, 27 Jan 2012 11:05:25 +0800 From: Wu Fengguang To: Andrew Morton cc: Andi Kleen , Wu Fengguang cc: Linux Memory Management List , Cc: LKML Subject: [PATCH 1/9] readahead: make context readahead more conservative References: <20120127030524.854259561@intel.com> Content-Disposition: inline; filename=readahead-context-tt Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Try to prevent negatively impact 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. Tested-by: Tao Ma Signed-off-by: Wu Fengguang --- mm/readahead.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) --- linux-next.orig/mm/readahead.c 2012-01-25 15:57:47.000000000 +0800 +++ linux-next/mm/readahead.c 2012-01-25 15:57:49.000000000 +0800 @@ -369,10 +369,10 @@ 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; /* @@ -383,8 +383,8 @@ 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; }