On Mon, 2016-09-19 at 16:11 +0900, Minchan Kim wrote: > Hi Tim, > > On Tue, Sep 13, 2016 at 11:52:27PM +0000, Chen, Tim C wrote: > > > > > > > > > > > > > > > > > - Avoid CPU time for splitting, collapsing THP across swap out/in. > > > Yes, if you want, please give us how bad it is. > > > > > It could be pretty bad. In an experiment with THP turned on and we > > enter swap, 50% of the cpu are spent in the page compaction path. > It's page compaction overhead, especially, pageblock_pfn_to_page. > Why is it related to overhead THP split for swapout? > I don't understand.
Today you have to split a large page into 4K pages to swap it out. Then after you swap in all the 4K pages, you have to re-compact them back into a large page. If you can swap the large page out as a contiguous unit, and swap it back in as a single large page, the splitting and re-compaction back into a large page can be avoided. Tim