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

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