linux-kernel - Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation

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Date:   Mon, 19 Feb 2018 09:47:35 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     robert.m.harris@...cle.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kemi Wang <kemi.wang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Yafang Shao <laoar.shao@...il.com>,
        Kangmin Park <l4stpr0gr4m@...il.com>,
        Yisheng Xie <xieyisheng1@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Vinayak Menon <vinmenon@...eaurora.org>
Subject: Re: [PATCH 1/1] mm, compaction: correct the bounds of
 __fragmentation_index()

On Sun, Feb 18, 2018 at 04:47:55PM +0000, robert.m.harris@...cle.com wrote:
> From: "Robert M. Harris" <robert.m.harris@...cle.com>
> 
> __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 <robert.m.harris@...cle.com>

You have to update sysctl_extfrag_threshold as well for the new bounds.
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.

-- 
Mel Gorman
SUSE Labs