Re: [RFC PATCH 0/5] pro-active compaction

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Jan 13, 2017 at 10:24:21AM +0100, Michal Hocko wrote:
> On Fri 13-01-17 16:14:28, Joonsoo Kim wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > 
> > Hello,
> > 
> > This is a patchset for pro-active compaction to reduce fragmentation.
> > It is a just RFC patchset so implementation detail isn't good.
> > I submit this for people who want to check the effect of pro-active
> > compaction.
> > 
> > Patch 1 ~ 4 introduces new metric for checking fragmentation. I think
> > that this new metric is useful to check fragmentation state
> > regardless of usefulness of pro-active compaction. Please let me know
> > if someone see that this new metric is useful. I'd like to submit it,
> > separately.
> 
> Could you describe this metric from a high level POV please?

There is some information at description on patch #3.

Anyway, in summary, it is an exponential moving average of unusable free
index which already exists. Unusable free index means the freepage
ratio at the moment that cannot be usable for specific order.
It is easy to understand if you see below equation.

unusable_free_index(order N) = 1 -
  (Number of freepages higher or equal than order N / Total freepages)

Thanks.

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