From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ua0-f198.google.com (mail-ua0-f198.google.com [209.85.217.198]) by kanga.kvack.org (Postfix) with ESMTP id 9C2BE6B0008 for ; Sun, 18 Feb 2018 11:46:07 -0500 (EST) Received: by mail-ua0-f198.google.com with SMTP id x22so3387209uaj.12 for ; Sun, 18 Feb 2018 08:46:07 -0800 (PST) Received: from aserp2130.oracle.com (aserp2130.oracle.com. [141.146.126.79]) by mx.google.com with ESMTPS id 66si2773229vka.384.2018.02.18.08.46.06 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 18 Feb 2018 08:46:06 -0800 (PST) From: robert.m.harris@oracle.com Subject: [PATCH 0/1] mm, compaction: correct the bounds of __fragmentation_index() Date: Sun, 18 Feb 2018 16:47:54 +0000 Message-Id: <1518972475-11340-1-git-send-email-robert.m.harris@oracle.com> Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Cc: Jonathan Corbet , Andrew Morton , Michal Hocko , Vlastimil Babka , "Kirill A. Shutemov" , Johannes Weiner , Kemi Wang , David Rientjes , Yafang Shao , Kangmin Park , Mel Gorman , Yisheng Xie , Davidlohr Bueso , Greg Kroah-Hartman , Huang Ying , Vinayak Menon , "Robert M. Harris" 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 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. Here's an illustration using a zone that I fragmented with selective calls to __alloc_pages() and __free_pages --- the fragmentation for order-1 could not be minimised further yet is reported as 0.5: # head -1 /proc/buddyinfo Node 0, zone DMA 1983 0 0 0 0 0 0 0 0 0 0 # head -1 /sys/kernel/debug/extfrag/extfrag_index Node 0, zone DMA -1.000 0.500 0.750 0.875 0.937 0.969 0.984 0.992 0.996 0.998 0.999 # With extreme memory shortage the reported fragmentation index does go lower. In fact, it can go below zero: # head -1 /proc/buddyinfo Node 0, zone DMA 1 0 0 0 0 0 0 0 0 0 0 # head -1 /sys/kernel/debug/extfrag/extfrag_index Node 0, zone DMA -1.000 0.-500 0.-250 0.-125 0.-62 0.-31 0.-15 0.-07 0.-03 0.-01 0.000 # 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. With this patch in place, the same two tests yield # head -1 /proc/buddyinfo Node 0, zone DMA 1983 0 0 0 0 0 0 0 0 0 0 # head -1 /sys/kernel/debug/extfrag/extfrag_index Node 0, zone DMA -1.000 0.000 0.500 0.750 0.875 0.937 0.969 0.984 0.992 0.996 0.998 # and # head -1 /proc/buddyinfo Node 0, zone DMA 1 0 0 0 0 0 0 0 0 0 0 # head -1 /sys/kernel/debug/extfrag/extfrag_index Node 0, zone DMA -1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 # Robert M. Harris (1): mm, compaction: correct the bounds of __fragmentation_index() Documentation/sysctl/vm.txt | 2 +- mm/compaction.c | 2 +- mm/vmstat.c | 47 +++++++++++++++++++++++++++++++++++---------- 3 files changed, 39 insertions(+), 12 deletions(-) -- 1.8.3.1 -- 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