From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751563AbcHPGff (ORCPT ); Tue, 16 Aug 2016 02:35:35 -0400 Received: from LGEAMRELO12.lge.com ([156.147.23.52]:40173 "EHLO lgeamrelo12.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751006AbcHPGfe (ORCPT ); Tue, 16 Aug 2016 02:35:34 -0400 X-Original-SENDERIP: 156.147.1.127 X-Original-MAILFROM: iamjoonsoo.kim@lge.com X-Original-SENDERIP: 10.177.222.138 X-Original-MAILFROM: iamjoonsoo.kim@lge.com Date: Tue, 16 Aug 2016 15:41:04 +0900 From: Joonsoo Kim To: Vlastimil Babka Cc: Andrew Morton , Michal Hocko , Mel Gorman , David Rientjes , Rik van Riel , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v6 08/11] mm, compaction: create compact_gap wrapper Message-ID: <20160816064104.GG17448@js1304-P5Q-DELUXE> References: <20160810091226.6709-1-vbabka@suse.cz> <20160810091226.6709-9-vbabka@suse.cz> <20160816061518.GE17448@js1304-P5Q-DELUXE> <656fea7f-753d-df56-744a-50b90f9a3842@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <656fea7f-753d-df56-744a-50b90f9a3842@suse.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 16, 2016 at 08:15:36AM +0200, Vlastimil Babka wrote: > On 08/16/2016 08:15 AM, Joonsoo Kim wrote: > >On Wed, Aug 10, 2016 at 11:12:23AM +0200, Vlastimil Babka wrote: > >>--- a/include/linux/compaction.h > >>+++ b/include/linux/compaction.h > >>@@ -58,6 +58,22 @@ enum compact_result { > >> > >> struct alloc_context; /* in mm/internal.h */ > >> > >>+/* > >>+ * Number of free order-0 pages that should be available above given watermark > >>+ * to make sure compaction has reasonable chance of not running out of free > >>+ * pages that it needs to isolate as migration target during its work. > >>+ */ > >>+static inline unsigned long compact_gap(unsigned int order) > >>+{ > >>+ /* > >>+ * Although all the isolations for migration are temporary, compaction > >>+ * may have up to 1 << order pages on its list and then try to split > >>+ * an (order - 1) free page. At that point, a gap of 1 << order might > >>+ * not be enough, so it's safer to require twice that amount. > >>+ */ > >>+ return 2UL << order; > >>+} > > > >I agree with this wrapper function but there is a question. > > > >Could you elaborate more on this code comment? Freescanner could keep > >COMPACT_CLUSTER_MAX freepages on the list. It's not associated with > >requested order at least for now. Why compact_gap is 2UL << order in > >this case? > > It's true that for high enough order, COMPACT_CLUSTER_MAX might be > more limiting than 1 << order. But then it also helps to have more AFAIK, regardless of order, migration scanner isolates COMPACT_CLUSTER_MAX pages. And, freepage scanner isolates more than nr_migratepages freepages. > free pages for probability of compaction success, so I don't think > it's worth complicating the compact_gap() formula. I agree that it's not worth complicating the compact_gap() formula but it would be better to fix the comment? Thanks.