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

[Minchan Kim]

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.

> So if we could deal with units of large page for swap, the splitting
> and compaction of ordinary pages to large page overhead could be avoided.
> 
>    51.89%    51.89%            :1688  [kernel.kallsyms]   [k] pageblock_pfn_to_page                       
>                       |
>                       --- pageblock_pfn_to_page
>                          |          
>                          |--64.57%-- compaction_alloc
>                          |          migrate_pages
>                          |          compact_zone
>                          |          compact_zone_order
>                          |          try_to_compact_pages
>                          |          __alloc_pages_direct_compact
>                          |          __alloc_pages_nodemask
>                          |          alloc_pages_vma
>                          |          do_huge_pmd_anonymous_page
>                          |          handle_mm_fault
>                          |          __do_page_fault
>                          |          do_page_fault
>                          |          page_fault
>                          |          0x401d9a
>                          |          
>                          |--34.62%-- compact_zone
>                          |          compact_zone_order
>                          |          try_to_compact_pages
>                          |          __alloc_pages_direct_compact
>                          |          __alloc_pages_nodemask
>                          |          alloc_pages_vma
>                          |          do_huge_pmd_anonymous_page
>                          |          handle_mm_fault
>                          |          __do_page_fault
>                          |          do_page_fault
>                          |          page_fault
>                          |          0x401d9a
>                           --0.81%-- [...]
> 
> Tim