From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with ESMTP id 9226F6B004A for ; Wed, 29 Sep 2010 10:42:24 -0400 (EDT) Date: Wed, 29 Sep 2010 15:41:59 +0100 From: Mel Gorman Subject: Re: zone state overhead Message-ID: <20100929144159.GC14204@csn.ul.ie> References: <20100928050801.GA29021@sli10-conroe.sh.intel.com> <20100928133059.GL8187@csn.ul.ie> <20100929100307.GA14204@csn.ul.ie> <20100929141730.GB14204@csn.ul.ie> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: Christoph Lameter Cc: David Rientjes , Shaohua Li , linux-mm@kvack.org List-ID: On Wed, Sep 29, 2010 at 09:34:09AM -0500, Christoph Lameter wrote: > On Wed, 29 Sep 2010, Mel Gorman wrote: > > > > Updating the threshold also is expensive. > > > > Even if it's moved to a read-mostly part of the zone such as after > > lowmem_reserve? > > The threshold is stored in the hot part of the per cpu page structure. > And the consequences of moving it? In terms of moving, it would probably work out better to move percpu_drift_mark after the lowmem_reserve and put the threshold after it so they're at least similarly hot across CPUs. > > > I thought more along the lines > > > of reducing the threshold for good if the VM runs into reclaim trouble > > > because of too high fuzziness in the counters. > > > > > > > That would be unfortunate as it would only take trouble to happen once > > for performance to be impaired for the remaining uptime of the machine. > > Reclaim also impairs performance and inaccurate counters may cause > unnecessary reclaim. Ah, it's limited to be fair. You might end up reclaiming "maximum drift" number of pages you didn't need to but that doesn't seem as bad. > Ultimately this is a tradeoff. The current thresholds > were calculated so that there will be zero impact even for very large > configurations where all processors continual page fault. I think we have > some leeway to go lower there. The tuning situation was a bit extreme. > Ok. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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/ . Don't email: email@kvack.org