From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f71.google.com (mail-wm0-f71.google.com [74.125.82.71]) by kanga.kvack.org (Postfix) with ESMTP id E88BE6B0005 for ; Mon, 19 Feb 2018 08:10:29 -0500 (EST) Received: by mail-wm0-f71.google.com with SMTP id c188so708065wma.7 for ; Mon, 19 Feb 2018 05:10:29 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id w190si6253270wmd.61.2018.02.19.05.10.28 for (version=TLS1 cipher=AES128-SHA bits=128/128); Mon, 19 Feb 2018 05:10:28 -0800 (PST) Date: Mon, 19 Feb 2018 13:10:24 +0000 From: Mel Gorman Subject: Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index() Message-ID: <20180219131024.oqonm6ba3pl2l4qa@suse.de> References: <1518972475-11340-1-git-send-email-robert.m.harris@oracle.com> <1518972475-11340-2-git-send-email-robert.m.harris@oracle.com> <20180219094735.g4sm4kxawjnojgyd@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Robert Harris Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Jonathan Corbet , Andrew Morton , Michal Hocko , Vlastimil Babka , "Kirill A. Shutemov" , Johannes Weiner , Kemi Wang , David Rientjes , Yafang Shao , Kangmin Park , Yisheng Xie , Davidlohr Bueso , Greg Kroah-Hartman , Huang Ying , Vinayak Menon On Mon, Feb 19, 2018 at 12:26:39PM +0000, Robert Harris wrote: > > > > On 19 Feb 2018, at 09:47, Mel Gorman wrote: > > > > On Sun, Feb 18, 2018 at 04:47:55PM +0000, robert.m.harris@oracle.com wrote: > >> From: "Robert M. Harris" > >> > >> __fragmentation_index() calculates a value used to determine whether > >> compaction should be favoured over page reclaim in the event of allocation > >> failure. The calculation itself is opaque and, on inspection, does not > >> match its existing description. The function purports to return a value > >> between 0 and 1000, representing units of 1/1000. Barring the case of a > >> pathological shortfall of memory, the lower bound is instead 500. This is > >> significant because it is the default value of sysctl_extfrag_threshold, > >> i.e. the value below which compaction should be avoided in favour of page > >> reclaim for costly pages. > >> > >> This patch implements and documents a modified version of the original > >> expression that returns a value in the range 0 <= index < 1000. It amends > >> the default value of sysctl_extfrag_threshold to preserve the existing > >> behaviour. > >> > >> Signed-off-by: Robert M. Harris > > > > You have to update sysctl_extfrag_threshold as well for the new bounds. > > This patch makes its default value zero. > Sorry, I'm clearly blind. > > It effectively makes it a no-op but it was a no-op already and adjusting > > that default should be supported by data indicating it's safe. > > Would it be acceptable to demonstrate using tracing that in both the > pre- and post-patch cases > > 1. compaction is attempted regardless of fragmentation index, > excepting that > > 2. reclaim is preferred even for non-zero fragmentation during > an extreme shortage of memory > If you can demonstrate that for both reclaim-intensive and compaction-intensive workloads then yes. Also include the reclaim and compaction stats from /proc/vmstat and not just tracepoints to demonstrate that reclaim doesn't get out of control and reclaim the world in response to failed high-order allocations such as THP. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org