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

Robert Harris
2018-02-19T12:26:39Z

> 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