From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755911AbaBEUwf (ORCPT ); Wed, 5 Feb 2014 15:52:35 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:38217 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752089AbaBEUwc (ORCPT ); Wed, 5 Feb 2014 15:52:32 -0500 Date: Wed, 5 Feb 2014 12:52:30 -0800 From: Andrew Morton To: Vladimir Davydov 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 Message-Id: <20140205125230.e1705369abcb634ddf141008@linux-foundation.org> In-Reply-To: <52F1E561.8020804@parallels.com> References: <4e2efebe688e06574f6495c634ac45d799e1518d.1389982079.git.vdavydov@parallels.com> <20140204135836.05c09c765073513e62edd174@linux-foundation.org> <52F1E561.8020804@parallels.com> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). > 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. David, you touched it last. Any hints?