From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752764AbcGMIhg (ORCPT ); Wed, 13 Jul 2016 04:37:36 -0400 Received: from outbound-smtp02.blacknight.com ([81.17.249.8]:42345 "EHLO outbound-smtp02.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751013AbcGMIha (ORCPT ); Wed, 13 Jul 2016 04:37:30 -0400 Date: Wed, 13 Jul 2016 09:37:26 +0100 From: Mel Gorman To: Johannes Weiner Cc: Andrew Morton , Linux-MM , Rik van Riel , Vlastimil Babka , Minchan Kim , Joonsoo Kim , LKML Subject: Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the highest requested zone Message-ID: <20160713083726.GE9806@techsingularity.net> References: <1467970510-21195-1-git-send-email-mgorman@techsingularity.net> <1467970510-21195-7-git-send-email-mgorman@techsingularity.net> <20160712140504.GC5881@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20160712140504.GC5881@cmpxchg.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 12, 2016 at 10:05:04AM -0400, Johannes Weiner wrote: > On Fri, Jul 08, 2016 at 10:34:42AM +0100, Mel Gorman wrote: > > kswapd checks all eligible zones to see if they need balancing even if it > > was woken for a lower zone. This made sense when we reclaimed on a > > per-zone basis because we wanted to shrink zones fairly so avoid > > age-inversion problems. Ideally this is completely unnecessary when > > reclaiming on a per-node basis. In theory, there may still be anomalies > > when all requests are for lower zones and very old pages are preserved in > > higher zones but this should be the exceptional case. > > > > Signed-off-by: Mel Gorman > > Acked-by: Vlastimil Babka > > Acked-by: Johannes Weiner > > I wasn't quite sure at first what the rationale is for this patch, > since it probably won't make much difference in pratice. Possibly not, it depends on how much embedded 32-bit platforms use features like zswap. What I wanted to avoid was a lowmem allocation for zswap excessively reclaiming highmem putting even further pressure on zswap if the pages are anonymous. > But I do > agree that the code is cleaner to have kswapd check exactly what it > was asked to check, rather than some do-the-"right"-thing magic. > But this is a justification on its own. I encountered an astonishing number of magic number handling that just happened to mostly work. I wanted to iron them out as much as possible. > A hypothetical onslaught of low-zone allocations will wreak havoc to > the page age in higher zones anyway, right? So I don't think that case > matters all that much. Possibly not, but it was straight-forward to mitigate the damage without too many side-effects. -- Mel Gorman SUSE Labs