From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx173.postini.com [74.125.245.173]) by kanga.kvack.org (Postfix) with SMTP id 475116B0044 for ; Thu, 20 Dec 2012 06:12:14 -0500 (EST) Date: Thu, 20 Dec 2012 11:12:08 +0000 From: Mel Gorman Subject: Re: [PATCH] mm: do not sleep in balance_pgdat if there's no i/o congestion Message-ID: <20121220111208.GD10819@suse.de> References: <50D24AF3.1050809@iskon.hr> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <50D24AF3.1050809@iskon.hr> Sender: owner-linux-mm@kvack.org List-ID: To: Zlatko Calusic Cc: Linus Torvalds , Andrew Morton , Hugh Dickins , linux-mm , Linux Kernel Mailing List On Thu, Dec 20, 2012 at 12:17:07AM +0100, Zlatko Calusic wrote: > On a 4GB RAM machine, where Normal zone is much smaller than > DMA32 zone, the Normal zone gets fragmented in time. This requires > relatively more pressure in balance_pgdat to get the zone above the > required watermark. Unfortunately, the congestion_wait() call in there > slows it down for a completely wrong reason, expecting that there's > a lot of writeback/swapout, even when there's none (much more common). > After a few days, when fragmentation progresses, this flawed logic > translates to a very high CPU iowait times, even though there's no > I/O congestion at all. If THP is enabled, the problem occurs sooner, > but I was able to see it even on !THP kernels, just by giving it a bit > more time to occur. > > The proper way to deal with this is to not wait, unless there's > congestion. Thanks to Mel Gorman, we already have the function that > perfectly fits the job. The patch was tested on a machine which > nicely revealed the problem after only 1 day of uptime, and it's been > working great. > --- > mm/vmscan.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > Acked-by: Mel Gorman email@kvack.org