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

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Date:   Tue, 13 Sep 2016 23:52:27 +0000
From:   "Chen, Tim C" <tim.c.chen@...el.com>
To:     Minchan Kim <minchan@...nel.org>,
        "Huang, Ying" <ying.huang@...el.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>,
        "Lu, Aaron" <aaron.lu@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vladimir Davydov <vdavydov@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: RE: [PATCH -v3 00/10] THP swap: Delay splitting THP during swapping
 out

>>
>> - 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.  
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