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From: "Chen, Tim C" <tim.c.chen@intel.com>
Subject: RE: [PATCH -v3 00/10] THP swap: Delay splitting THP during swapping
 out
Date: Tue, 13 Sep 2016 23:52:27 +0000
Message-ID: <045D8A5597B93E4EBEDDCBF1FC15F50935BF9343@fmsmsx104.amr.corp.intel.com>
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To: Minchan Kim <minchan@kernel.org>, "Huang, Ying" <ying.huang@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, "Hansen, Dave" <dave.hansen@intel.com>, "Kleen, Andi" <andi.kleen@intel.com>, "Lu, Aaron" <aaron.lu@intel.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Hugh Dickins <hughd@google.com>, Shaohua Li <shli@kernel.org>, Rik van Riel <riel@redhat.com>, Andrea Arcangeli <aarcange@redhat.com>, "Kirill A .
 Shutemov" <kirill.shutemov@linux.intel.com>, Vladimir Davydov <vdavydov@virtuozzo.com>, Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>

>>
>> - 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. =20
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                      =20
                      |
                      --- pageblock_pfn_to_page
                         |         =20
                         |--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
                         |         =20
                         |--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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