Re: [PATCH -v3 00/10] THP swap: Delay splitting THP during swapping out

[Tim Chen]

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.ÂÂIn an experiment with THP turned on and we
> > enter swap, 50% of the cpu are spent in the page compaction path.ÂÂ
> 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