Re: [PATCH 1/3] mm/page_alloc: don't break highest order freepage if steal

[Vlastimil Babka]

On 04/27/2015 10:42 AM, Joonsoo Kim wrote:
> On Mon, Apr 27, 2015 at 09:08:50AM +0100, Mel Gorman wrote:
> > On Mon, Apr 27, 2015 at 04:23:39PM +0900, Joonsoo Kim wrote:
> > > When we steal whole pageblock, we don't need to break highest order
> > > freepage. Perhaps, there is small order freepage so we can use it.
> >
> > The reason why the largest block is taken is to reduce the probability
> > there will be another fallback event in the near future. Early on, there
> > were a lot of tests conducted to measure the number of external fragmenting
> > events and take steps to reduce them. Stealing the largest highest order
> > freepage was one of those steps.
>
> Hello, Mel.
>
> Purpose of this patch is not "stop steal highest order freepage".
> Currently, in case of that we steal all freepage including highest
> order freepage in certain pageblock, we break highest order freepage and
> return it even if we have low order freepage that we immediately steal.
>
> For example,
>
> Pageblock A has 5 freepage (4 * order 0, 1 * order 3) and
> we try to steal all freepage on pageblock A.
>
> Withouth this patch, we move all freepage to requested migratetype
> buddy list and break order 3 freepage. Leftover is like as following.
>
> (5 * order 0, 1 * order 1, 1* order 2)
>
> With this patch, (3 * order 0, 1 * order 3) remains.
>
> I think that this is better than before because we still have high order
> page. Isn't it?

I agree that this should be better in some cases and shouldn't be worse 
in any case. Nice catch.

> Thanks.
>
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