From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id 562036B004A for ; Wed, 29 Sep 2010 10:46:36 -0400 (EDT) Date: Wed, 29 Sep 2010 15:45:56 +0100 From: Mel Gorman Subject: Re: zone state overhead Message-ID: <20100929144556.GD14204@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> <20100929144159.GC14204@csn.ul.ie> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20100929144159.GC14204@csn.ul.ie> 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 03:41:59PM +0100, Mel Gorman wrote: > 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 should be clearer here. Initially, I'm thinking the consequences of moving it are not terrible bad so I'm wondering if you see some problem I have not thought of. If the threshold value is sharing the cache line with watermark or lowmem_reserve, then it should still have the same hotness in the path we really care about (zone_watermark_ok for example) without necessarily needing to be part of the per-cpu structure. The real badness would be if an additional cache line was required due to the move but I don't think this is the case (but I didn't double check with pahole or the back of an envelope either). The line will be dirtied and cause a bounce when kswapd wakes or goes to sleep but this should not be a severe problem. -- 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