From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1753360AbdBNQfT (ORCPT <rfc822;w@1wt.eu>);
        Tue, 14 Feb 2017 11:35:19 -0500
Received: from gum.cmpxchg.org ([85.214.110.215]:35260 "EHLO gum.cmpxchg.org"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1753126AbdBNQfH (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 14 Feb 2017 11:35:07 -0500
Date: Tue, 14 Feb 2017 11:34:53 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org, Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        David Rientjes <rientjes@google.com>,
        Mel Gorman <mgorman@techsingularity.net>, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH v2 02/10] mm, compaction: remove redundant watermark
 check in compact_finished()
Message-ID: <20170214163453.GC2450@cmpxchg.org>
References: <20170210172343.30283-1-vbabka@suse.cz>
 <20170210172343.30283-3-vbabka@suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170210172343.30283-3-vbabka@suse.cz>
User-Agent: Mutt/1.7.2 (2016-11-26)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Feb 10, 2017 at 06:23:35PM +0100, Vlastimil Babka wrote:
> When detecting whether compaction has succeeded in forming a high-order page,
> __compact_finished() employs a watermark check, followed by an own search for
> a suitable page in the freelists. This is not ideal for two reasons:
> 
> - The watermark check also searches high-order freelists, but has a less strict
>   criteria wrt fallback. It's therefore redundant and waste of cycles. This was
>   different in the past when high-order watermark check attempted to apply
>   reserves to high-order pages.
> 
> - The watermark check might actually fail due to lack of order-0 pages.
>   Compaction can't help with that, so there's no point in continuing because of
>   that. It's possible that high-order page still exists and it terminates.
> 
> This patch therefore removes the watermark check. This should save some cycles
> and terminate compaction sooner in some cases.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>