On 19 Feb 2018, at 08:24, Michal Hocko <mhocko@kernel.org> wrote:

On Sun 18-02-18 16:47:54, 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 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:

Cover letter for a single patch is usually an overkill. Why is this
information not valuable in the patch description directly?