From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta7.messagelabs.com (mail6.bemta7.messagelabs.com [216.82.255.55]) by kanga.kvack.org (Postfix) with ESMTP id 3437F6B010D for ; Wed, 29 Jun 2011 06:58:28 -0400 (EDT) Message-ID: <4E0B052C.7000500@draigBrady.com> Date: Wed, 29 Jun 2011 11:57:48 +0100 From: =?ISO-8859-1?Q?P=E1draig_Brady?= MIME-Version: 1.0 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> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Mel Gorman , James Bottomley , Colin King , Minchan Kim , Andrew Lutomirski , Rik van Riel , Johannes Weiner , linux-mm , linux-kernel 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: Padraig 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, Padraig. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org