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From:   <alexlzhu@fb.com>
To:     <linux-mm@kvack.org>
CC:     <willy@infradead.org>, <hannes@cmpxchg.org>,
        <akpm@linux-foundation.org>, <riel@surriel.com>,
        <kernel-team@fb.com>, <linux-kernel@vger.kernel.org>,
        Alexander Zhu <alexlzhu@fb.com>
Subject: [RFC 0/3] THP Shrinker
Date:   Thu, 25 Aug 2022 14:30:51 -0700
Message-ID: <cover.1661461643.git.alexlzhu@fb.com>
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From: Alexander Zhu <alexlzhu@fb.com>

Transparent Hugepages use a larger page size of 2MB in comparison to
normal sized pages that are 4kb. A larger page size allows for fewer TLB
cache misses and thus more efficient use of the CPU. Using a larger page
size also results in more memory waste, which can hurt performance in som=
e
use cases. THPs are currently enabled in the Linux Kernel by applications
in limited virtual address ranges via the madvise system call.  The THP
shrinker tries to find a balance between increased use of THPs, and
increased use of memory. It shrinks the size of memory by removing the
underutilized THPs that are identified by the thp_utilization scanner.=20

In our experiments we have noticed that the least utilized THPs are almos=
t
entirely unutilized.

Sample Output:=20

Utilized[0-50]: 1331 680884
Utilized[51-101]: 9 3983
Utilized[102-152]: 3 1187
Utilized[153-203]: 0 0
Utilized[204-255]: 2 539
Utilized[256-306]: 5 1135
Utilized[307-357]: 1 192
Utilized[358-408]: 0 0
Utilized[409-459]: 1 57
Utilized[460-512]: 400 13
Last Scan Time: 223.98
Last Scan Duration: 70.65

Above is a sample obtained from one of our test machines when THP is alwa=
ys
enabled. Of the 1331 THPs in this thp_utilization sample that have from
0-50 utilized subpages, we see that there are 680884 free pages. This
comes out to 680884 / (512 * 1331) =3D 99.91% zero pages in the least
utilized bucket. This represents 680884 * 4KB =3D 2.7GB memory waste.

Also note that the vast majority of pages are either in the least utilize=
d
[0-50] or most utilized [460-512] buckets. The least utilized THPs are=20
responsible for almost all of the memory waste when THP is always=20
enabled. Thus by clearing out THPs in the lowest utilization bucket
we extract most of the improvement in CPU efficiency. We have seen=20
similar results on our production hosts.

This patchset introduces the THP shrinker we have developed to identify
and split the least utilized THPs. It includes the thp_utilization=20
changes that groups anonymous THPs into buckets, the split_huge_page()
changes that identify and zap zero 4KB pages within THPs and the shrinker
changes. It should be noted that the split_huge_page() changes are based
off previous work done by Yu Zhao.=20

In the future, we intend to allow additional tuning to the shrinker
based on workload depending on CPU/IO/Memory pressure and the=20
amount of anonymous memory. The long term goal is to eventually always=20
enable THP for all applications and deprecate madvise entirely.

Alexander Zhu (3):
  mm: add thp_utilization metrics to debugfs
  mm: changes to split_huge_page() to free zero filled tail pages
  mm: THP low utilization shrinker

 Documentation/admin-guide/mm/transhuge.rst    |   9 +
 include/linux/huge_mm.h                       |   9 +
 include/linux/list_lru.h                      |  24 ++
 include/linux/mm_types.h                      |   5 +
 include/linux/rmap.h                          |   2 +-
 include/linux/vm_event_item.h                 |   2 +
 mm/huge_memory.c                              | 333 +++++++++++++++++-
 mm/list_lru.c                                 |  49 +++
 mm/migrate.c                                  |  60 +++-
 mm/migrate_device.c                           |   4 +-
 mm/page_alloc.c                               |   6 +
 mm/vmstat.c                                   |   2 +
 .../selftests/vm/split_huge_page_test.c       |  58 ++-
 tools/testing/selftests/vm/vm_util.c          |  23 ++
 tools/testing/selftests/vm/vm_util.h          |   1 +
 15 files changed, 569 insertions(+), 18 deletions(-)

--=20
2.30.2