Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754910Ab1F2K63 (ORCPT ); Wed, 29 Jun 2011 06:58:29 -0400 Received: from mail1.slb.deg.dub.stisp.net ([84.203.253.98]:2951 "HELO mail1.slb.deg.dub.stisp.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1754034Ab1F2K61 (ORCPT ); Wed, 29 Jun 2011 06:58:27 -0400 Message-ID: <4E0B052C.7000500@draigBrady.com> Date: Wed, 29 Jun 2011 11:57:48 +0100 From: =?ISO-8859-1?Q?P=E1draig_Brady?= User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3 MIME-Version: 1.0 To: Andrew Morton CC: Mel Gorman , James Bottomley , Colin King , Minchan Kim , Andrew Lutomirski , Rik van Riel , Johannes Weiner , linux-mm , linux-kernel Subject: Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely References: <1308926697-22475-1-git-send-email-mgorman@suse.de> <1308926697-22475-2-git-send-email-mgorman@suse.de> <20110628144900.b33412c6.akpm@linux-foundation.org> In-Reply-To: <20110628144900.b33412c6.akpm@linux-foundation.org> X-Enigmail-Version: 1.0.1 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2489 Lines: 72 On 28/06/11 22:49, Andrew Morton wrote: > On Fri, 24 Jun 2011 15:44:54 +0100 > Mel Gorman wrote: > >> During allocator-intensive workloads, kswapd will be woken frequently >> causing free memory to oscillate between the high and min watermark. >> This is expected behaviour. >> >> A problem occurs if the highest zone is small. balance_pgdat() >> only considers unreclaimable zones when priority is DEF_PRIORITY >> but sleeping_prematurely considers all zones. It's possible for this >> sequence to occur >> >> 1. kswapd wakes up and enters balance_pgdat() >> 2. At DEF_PRIORITY, marks highest zone unreclaimable >> 3. At DEF_PRIORITY-1, ignores highest zone setting end_zone >> 4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from >> highest zone, clearing all_unreclaimable. Highest zone >> is still unbalanced >> 5. kswapd returns and calls sleeping_prematurely >> 6. sleeping_prematurely looks at *all* zones, not just the ones >> being considered by balance_pgdat. The highest small zone >> has all_unreclaimable cleared but but the zone is not >> balanced. all_zones_ok is false so kswapd stays awake >> >> This patch corrects the behaviour of sleeping_prematurely to check >> the zones balance_pgdat() checked. > > But kswapd is making progress: it's reclaiming slab. Eventually that > won't work any more and all_unreclaimable will not be cleared and the > condition will fix itself up? > > > > btw, > > if (!sleeping_prematurely(...)) > sleep(); > > hurts my brain. My brain would prefer > > if (kswapd_should_sleep(...)) > sleep(); > > no? > >> Reported-and-tested-by: P?draig Brady > > But what were the before-and-after observations? I don't understand > how this can cause a permanent cpuchew by kswapd. Context: http://marc.info/?t=130865025500001&r=1&w=2 https://bugzilla.redhat.com/show_bug.cgi?id=712019 Summary: This will spin kswapd0 on my SNB laptop with 3GB RAM (with small normal zone): dd bs=1M count=3000 if=/dev/zero of=spin.test Basically once a certain amount of data is cached, kswapd0 will start spinning, until the data is removed from cache (by `rm spin.test` for example). cheers, P?draig. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/