From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756804AbaBFSv2 (ORCPT ); Thu, 6 Feb 2014 13:51:28 -0500 Received: from relay.parallels.com ([195.214.232.42]:56656 "EHLO relay.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753039AbaBFSv1 (ORCPT ); Thu, 6 Feb 2014 13:51:27 -0500 Message-ID: <52F3D9A9.1070107@parallels.com> Date: Thu, 6 Feb 2014 22:51:21 +0400 From: Vladimir Davydov MIME-Version: 1.0 To: Andrew Morton CC: , , , Mel Gorman , Michal Hocko , Johannes Weiner , Rik van Riel , Dave Chinner , Glauber Costa Subject: Re: [PATCH 2/3] mm: vmscan: get rid of DEFAULT_SEEKS and document shrink_slab logic References: <4e2efebe688e06574f6495c634ac45d799e1518d.1389982079.git.vdavydov@parallels.com> <20140204135836.05c09c765073513e62edd174@linux-foundation.org> <52F1E561.8020804@parallels.com> <20140205125230.e1705369abcb634ddf141008@linux-foundation.org> In-Reply-To: <20140205125230.e1705369abcb634ddf141008@linux-foundation.org> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit X-Originating-IP: [81.5.110.170] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/06/2014 12:52 AM, Andrew Morton wrote: > On Wed, 5 Feb 2014 11:16:49 +0400 Vladimir Davydov wrote: > >>> So why did I originally make DEFAULT_SEEKS=2? Because I figured that to >>> recreate (say) an inode would require a seek to the inode data then a >>> seek back. Is it legitimate to include the >>> seek-back-to-what-you-were-doing-before seek in the cost of an inode >>> reclaim? I guess so... >> Hmm, that explains this 2. Since we typically don't need to "seek back" >> when recreating a cache page, as they are usually read in bunches by >> readahead, the number of seeks to bring back a user page is 1, while the >> number of seeks to recreate an average inode is 2, right? > Sounds right to me. > >> Then to scan inodes and user pages so that they would generate >> approximately the same number of seeks, we should calculate the number >> of objects to scan as follows: >> >> nr_objects_to_scan = nr_pages_scanned / lru_pages * >> nr_freeable_objects / >> shrinker->seeks >> >> where shrinker->seeks = DEFAULT_SEEKS = 2 for inodes. > hm, I wonder if we should take the size of the object into account. > Should we be maximizing (memory-reclaimed / seeks-to-reestablish-it). I'm not sure I understand you quite right. You mean that if two slab caches have obj sizes 1k and 2k and both of them need 2 seeks to recreate an object, we should scan the 1k (or 2k?) slab cache more aggressively than the 2k one? Hmm... I don't know. It depends on what we want to achieve. But this won't balance the seeks, which is our goal for now, IIUC. >> But currently we >> have four times that. I can explain why we should multiply this by 2 - >> we do not count pages moving from active to inactive lrus in >> nr_pages_scanned, and 2*nr_pages_scanned can be a good approximation for >> that - but I have no idea why we multiply it by 4... > I don't understand this code at all: > > total_scan = nr; > delta = (4 * nr_pages_scanned) / shrinker->seeks; > delta *= freeable; > do_div(delta, lru_pages + 1); > total_scan += delta; > > If it actually makes any sense, it sorely sorely needs documentation. To find its roots I had to checkout the linux history tree: commit c3f4656118a78c1c294e0b4d338ac946265a822b Author: Andrew Morton Date: Mon Dec 29 23:48:44 2003 -0800 [PATCH] shrink_slab acounts for seeks incorrectly wli points out that shrink_slab inverts the sense of shrinker->seeks: those caches which require more seeks to reestablish an object are shrunk harder. That's wrong - they should be shrunk less. So fix that up, but scaling the result so that the patch is actually a no-op at this time, because all caches use DEFAULT_SEEKS (2). diff --git a/mm/vmscan.c b/mm/vmscan.c index b8594827bbac..f2da3c9fb346 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -154,7 +154,7 @@ static int shrink_slab(long scanned, unsigned int gfp_mask) list_for_each_entry(shrinker, &shrinker_list, list) { unsigned long long delta; - delta = scanned * shrinker->seeks; + delta = 4 * (scanned / shrinker->seeks); delta *= (*shrinker->shrinker)(0, gfp_mask); do_div(delta, pages + 1); shrinker->nr += delta; So the idea seemed to be fixing a bug without introducing any functional changes. Since then we have been living with this "4", which makes no sense (?). Nobody complained though. Thanks. > David, you touched it last. Any hints?