Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()

[Robert Harris <robert.m.harris@oracle.com>]

> On 19 Feb 2018, at 09:47, Mel Gorman <mgorman@suse.de> wrote:
> 
> On Sun, Feb 18, 2018 at 04:47:55PM +0000, robert.m.harris@oracle.com wrote:
>> From: "Robert M. Harris" <robert.m.harris@oracle.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@oracle.com>
> 
> 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

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