From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932074AbcBWUE0 (ORCPT ); Tue, 23 Feb 2016 15:04:26 -0500 Received: from gum.cmpxchg.org ([85.214.110.215]:59936 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755162AbcBWUEY (ORCPT ); Tue, 23 Feb 2016 15:04:24 -0500 Date: Tue, 23 Feb 2016 12:04:16 -0800 From: Johannes Weiner To: Mel Gorman Cc: Linux-MM , Rik van Riel , Vlastimil Babka , LKML Subject: Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Message-ID: <20160223200416.GA27563@cmpxchg.org> References: <1456239890-20737-1-git-send-email-mgorman@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1456239890-20737-1-git-send-email-mgorman@techsingularity.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 23, 2016 at 03:04:23PM +0000, Mel Gorman wrote: > In many benchmarks, there is an obvious difference in the number of > allocations from each zone as the fair zone allocation policy is removed > towards the end of the series. For example, this is the allocation stats > when running blogbench that showed no difference in headling performance > > mmotm-20160209 nodelru-v2 > DMA allocs 0 0 > DMA32 allocs 7218763 608067 > Normal allocs 12701806 18821286 > Movable allocs 0 0 According to the mmotm numbers, your DMA32 zone is over a third of available memory, yet in the nodelru-v2 kernel sees only 3% of the allocations. That's an insanely high level of aging inversion, where the lifetime of a cache entry is again highly dependent on placement. The fact that this doesn't make a performance difference in the specific benchmarks you ran only proves just that: these specific benchmarks don't care. IMO, benchmarking is not enough here. If this is truly supposed to be unproblematic, then I think we need a reasoned explanation. I can't imagine how it possibly could be, though. If reclaim can't guarantee a balanced zone utilization then the allocator has to keep doing it. :( As far as I'm concerned, the original reason for the fair zone allocator still applies.