From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f200.google.com (mail-qt0-f200.google.com [209.85.216.200]) by kanga.kvack.org (Postfix) with ESMTP id 916C86B0005 for ; Mon, 19 Feb 2018 07:27:12 -0500 (EST) Received: by mail-qt0-f200.google.com with SMTP id p11so8939215qtg.19 for ; Mon, 19 Feb 2018 04:27:12 -0800 (PST) Received: from userp2130.oracle.com (userp2130.oracle.com. [156.151.31.86]) by mx.google.com with ESMTPS id z16si3309901qta.419.2018.02.19.04.27.11 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 19 Feb 2018 04:27:11 -0800 (PST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 11.2 \(3445.5.20\)) Subject: Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index() From: Robert Harris In-Reply-To: <20180219094735.g4sm4kxawjnojgyd@suse.de> Date: Mon, 19 Feb 2018 12:26:39 +0000 Content-Transfer-Encoding: 7bit Message-Id: 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> Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman 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 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. > 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 ? Robert Harris -- 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