From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932317AbbLHBAn (ORCPT ); Mon, 7 Dec 2015 20:00:43 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:40534 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932072AbbLHBAm (ORCPT ); Mon, 7 Dec 2015 20:00:42 -0500 Date: Mon, 7 Dec 2015 17:00:41 -0800 From: Andrew Morton To: Joonsoo Kim Cc: Vlastimil Babka , Mel Gorman , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Joonsoo Kim Subject: Re: [PATCH] mm/compaction: restore COMPACT_CLUSTER_MAX to 32 Message-Id: <20151207170041.c470d362915ae1b42a8a4ef8@linux-foundation.org> In-Reply-To: <1449115900-20112-1-git-send-email-iamjoonsoo.kim@lge.com> References: <1449115900-20112-1-git-send-email-iamjoonsoo.kim@lge.com> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; 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 Thu, 3 Dec 2015 13:11:40 +0900 Joonsoo Kim wrote: > Until now, COMPACT_CLUSTER_MAX is defined as SWAP_CLUSTER_MAX. > Commit ("mm: increase SWAP_CLUSTER_MAX to batch TLB flushes") > changes SWAP_CLUSTER_MAX from 32 to 256 to improve tlb flush performance > so COMPACT_CLUSTER_MAX is also changed to 256. "mm: increase SWAP_CLUSTER_MAX to batch TLB flushes" has been in limbo for quite a while. Because it has been unclear whether the patch's benefits exceed its costs+risks. We should make a decision here - either do the appropriate testing or drop the patch. > But, it has > no justification on compaction-side and I think that loss is more than > benefit. > > One example is that migration scanner would isolates and migrates > too many pages unnecessarily with 256 COMPACT_CLUSTER_MAX. It may be > enough to migrate 4 pages in order to make order-2 page, but, now, > compaction will migrate 256 pages. > > To reduce this unneeded overhead, this patch restores > COMPACT_CLUSTER_MAX to 32. > > ... > > --- a/include/linux/swap.h > +++ b/include/linux/swap.h > @@ -155,7 +155,7 @@ enum { > }; > > #define SWAP_CLUSTER_MAX 256UL > -#define COMPACT_CLUSTER_MAX SWAP_CLUSTER_MAX > +#define COMPACT_CLUSTER_MAX 32UL > > /* > * Ratio between zone->managed_pages and the "gap" that above the per-zone