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

[Mel Gorman]

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

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