From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id B76E98D0041 for ; Thu, 3 Mar 2011 03:17:58 -0500 (EST) Message-Id: <20110303074950.979428322@intel.com> Date: Thu, 03 Mar 2011 14:45:23 +0800 From: Wu Fengguang Subject: [PATCH 18/27] writeback: enforce 1/4 gap between the dirty/background thresholds References: <20110303064505.718671603@intel.com> Content-Disposition: inline; filename=writeback-fix-oversize-background-thresh.patch Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Jan Kara , Peter Zijlstra , Wu Fengguang , Christoph Hellwig , Trond Myklebust , Dave Chinner , Theodore Ts'o , Chris Mason , Mel Gorman , Rik van Riel , KOSAKI Motohiro , Greg Thelen , Minchan Kim , Vivek Goyal , Andrea Righi , Balbir Singh , linux-mm , linux-fsdevel@vger.kernel.org, LKML The change is virtually a no-op for the majority users that use the default 10/20 background/dirty ratios. For others don't know why they are setting background ratio close enough to dirty ratio. Someone must set background ratio equal to dirty ratio, but no one seems to notice or complain that it's then silently halved under the hood.. The other solution is to return -EIO when setting a too large background threshold or a too small dirty threshold. However that could possibly break some disordered usage scenario, eg. echo 10 > /proc/sys/vm/dirty_ratio echo 5 > /proc/sys/vm/dirty_background_ratio The first echo will fail because the background ratio is still 10. Such order dependent behavior seems disgusting for end users. CC: Peter Zijlstra Signed-off-by: Wu Fengguang --- mm/page-writeback.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) --- linux-next.orig/mm/page-writeback.c 2011-03-02 17:04:16.000000000 +0800 +++ linux-next/mm/page-writeback.c 2011-03-02 17:06:17.000000000 +0800 @@ -422,8 +422,14 @@ void global_dirty_limits(unsigned long * else background = (dirty_background_ratio * available_memory) / 100; - if (background >= dirty) - background = dirty / 2; + /* + * Ensure at least 1/4 gap between background and dirty thresholds, so + * that when dirty throttling starts at (background + dirty)/2, it's + * below or at the entrance of the soft dirty throttle scope. + */ + if (background > dirty - dirty / (DIRTY_SCOPE / 2)) + background = dirty - dirty / (DIRTY_SCOPE / 2); + tsk = current; if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) { background += background / 4; -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org