From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753331Ab1GNBUJ (ORCPT ); Wed, 13 Jul 2011 21:20:09 -0400 Received: from fgwmail5.fujitsu.co.jp ([192.51.44.35]:37450 "EHLO fgwmail5.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752977Ab1GNBUI (ORCPT ); Wed, 13 Jul 2011 21:20:08 -0400 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 Message-ID: <4E1E444C.3090000@jp.fujitsu.com> Date: Thu, 14 Jul 2011 10:20:12 +0900 From: KOSAKI Motohiro User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; ja; rv:1.9.2.18) Gecko/20110616 Lightning/1.0b2 Thunderbird/3.1.11 MIME-Version: 1.0 To: mgorman@suse.de CC: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] mm: page allocator: Initialise ZLC for first zone eligible for zone_reclaim References: <1310389274-13995-1-git-send-email-mgorman@suse.de> <1310389274-13995-3-git-send-email-mgorman@suse.de> <4E1CF1A3.3050401@jp.fujitsu.com> <20110713110246.GF7529@suse.de> In-Reply-To: <20110713110246.GF7529@suse.de> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (2011/07/13 20:02), Mel Gorman wrote: > On Wed, Jul 13, 2011 at 10:15:15AM +0900, KOSAKI Motohiro wrote: >> (2011/07/11 22:01), Mel Gorman wrote: >>> The zonelist cache (ZLC) is used among other things to record if >>> zone_reclaim() failed for a particular zone recently. The intention >>> is to avoid a high cost scanning extremely long zonelists or scanning >>> within the zone uselessly. >>> >>> Currently the zonelist cache is setup only after the first zone has >>> been considered and zone_reclaim() has been called. The objective was >>> to avoid a costly setup but zone_reclaim is itself quite expensive. If >>> it is failing regularly such as the first eligible zone having mostly >>> mapped pages, the cost in scanning and allocation stalls is far higher >>> than the ZLC initialisation step. >>> >>> This patch initialises ZLC before the first eligible zone calls >>> zone_reclaim(). Once initialised, it is checked whether the zone >>> failed zone_reclaim recently. If it has, the zone is skipped. As the >>> first zone is now being checked, additional care has to be taken about >>> zones marked full. A zone can be marked "full" because it should not >>> have enough unmapped pages for zone_reclaim but this is excessive as >>> direct reclaim or kswapd may succeed where zone_reclaim fails. Only >>> mark zones "full" after zone_reclaim fails if it failed to reclaim >>> enough pages after scanning. >>> >>> Signed-off-by: Mel Gorman >> >> If I understand correctly this patch's procs/cons is, >> >> pros. >> 1) faster when zone reclaim doesn't work effectively >> > > Yes. > >> cons. >> 2) slower when zone reclaim is off > > How is it slower with zone_reclaim off? > > Before > > if (zone_reclaim_mode == 0) > goto this_zone_full; > ... > this_zone_full: > if (NUMA_BUILD) > zlc_mark_zone_full(zonelist, z); > if (NUMA_BUILD && !did_zlc_setup && nr_online_nodes > 1) { > ... > } > > After > if (NUMA_BUILD && !did_zlc_setup && nr_online_nodes > 1) { > ... > } > if (zone_reclaim_mode == 0) > goto this_zone_full; > this_zone_full: > if (NUMA_BUILD) > zlc_mark_zone_full(zonelist, z); > > Bear in mind that if the watermarks are met on the first zone, the zlc > setup does not occur. Right you are. thank you correct me. >> 3) slower when zone recliam works effectively >> > > Marginally slower. It's now calling zlc setup so once a second it's > zeroing a bitmap and calling zlc_zone_worth_trying() on the first > zone testing a bit on a cache-hot structure. > > As the ineffective case can be triggered by a simple cp, I think the > cost is justified. Can you think of a better way of doing this? So, now I'm revisit your number in [0/3]. and I've conclude your patch improve simple cp case too. then please forget my last mail. this patch looks nicer. Reviewed-by: KOSAKI Motohiro > >> (2) and (3) are frequently happen than (1), correct? > > Yes. I'd still expect zone_reclaim to be off on the majority of > machines and even when enabled, I think it's relatively rare we hit the > case where the workload is regularly falling over to the other node > except in the case where it's a file server. Still, a cp is not to > uncommon that the kernel should slow to a crawl as a result. > >> At least, I think we need to keep zero impact when zone reclaim mode is off. >> > > I agree with this but I'm missing where we are taking the big hit with > zone_reclaim==0. >