Messages in this thread | | | Date | Tue, 12 May 2015 09:57:00 +0200 | From | Vlastimil Babka <> | Subject | Re: [PATCH 1/3] mm/page_alloc: don't break highest order freepage if steal |
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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.
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