[PATCH v6 0/3] mm: ZSWAP swap-out of mTHP folios

Kanchana P Sridhar
2024-08-29T21:27:07Z

Hi All,

This patch-series enables zswap_store() to accept and store mTHP
folios. The most significant contribution in this series is from the
earlier RFC submitted by Ryan Roberts [1]. Ryan's original RFC has been
migrated to v6.11-rc3 in patch 2/4 of this series.

[1]: [RFC PATCH v1] mm: zswap: Store large folios without splitting
https://lore.kernel.org/linux-mm/20231019110543.3284654-1-ryan.roberts@arm.com/T/#u

Additionally, there is an attempt to modularize some of the functionality
in zswap_store(), to make it more amenable to supporting any-order
mTHPs. For instance, the function zswap_store_entry() stores a zswap_entry
in the xarray. Likewise, zswap_delete_stored_offsets() can be used to
delete all offsets corresponding to a higher order folio stored in zswap.

For accounting purposes, the patch-series adds per-order mTHP sysfs
"zswpout" counters that get incremented upon successful zswap_store of
an mTHP folio:

/sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/zswpout

A new config variable CONFIG_ZSWAP_STORE_THP_DEFAULT_ON (off by default)
will enable/disable zswap storing of (m)THP. When disabled, zswap will
fallback to rejecting the mTHP folio, to be processed by the backing
swap device.

This patch-series is a precursor to ZSWAP compress batching of mTHP
swap-out and decompress batching of swap-ins based on swapin_readahead(),
using Intel IAA hardware acceleration, which we would like to submit in
subsequent RFC patch-series, with performance improvement data.

Thanks to Ying Huang for pre-posting review feedback and suggestions!

Thanks also to Nhat, Yosry and Barry for their helpful feedback, data
reviews and suggestions!

Changes since v5:
=================
1) Rebased to mm-unstable as of 8/29/2024,
commit 9287e4adbc6ab8fa04d25eb82e097fed877a4642.
2) Added CONFIG_ZSWAP_STORE_THP_DEFAULT_ON (off by default) to
enable/disable zswap_store() of mTHP folios. Thanks Nhat for the
suggestion to add a knob by which users can enable/disable this
change. Nhat, I hope this is along the lines of what you were
thinking.
3) Added vm-scalability usemem data with 4K folios with
CONFIG_ZSWAP_STORE_THP_DEFAULT_ON off, that I gathered to make sure
there is no regression with this change.
4) Added data with usemem with 64K and 2M THP for an alternate view of
before/after, as suggested by Yosry, so we can understand the impact
of when mTHPs are split into 4K folios in shrink_folio_list()
(CONFIG_THP_SWAP off) vs. not split (CONFIG_THP_SWAP on) and stored
in zswap. Thanks Yosry for this suggestion.

Changes since v4:
=================
1) Published before/after data with zstd, as suggested by Nhat (Thanks
Nhat for the data reviews!).
2) Rebased to mm-unstable from 8/27/2024,
commit b659edec079c90012cf8d05624e312d1062b8b87.
3) Incorporated the change in memcontrol.h that defines obj_cgroup_get() if
CONFIG_MEMCG is not defined, to resolve build errors reported by kernel
robot; as per Nhat's and Michal's suggestion to not require a separate
patch to fix the build errors (thanks both!).
4) Deleted all same-filled folio processing in zswap_store() of mTHP, as
suggested by Yosry (Thanks Yosry!).
5) Squashed the commits that define new mthp zswpout stat counters, and
invoke count_mthp_stat() after successful zswap_store()s; into a single
commit. Thanks Yosry for this suggestion!

Changes since v3:
=================
1) Rebased to mm-unstable commit 8c0b4f7b65fd1ca7af01267f491e815a40d77444.
Thanks to Barry for suggesting aligning with Ryan Roberts' latest
changes to count_mthp_stat() so that it's always defined, even when THP
is disabled. Barry, I have also made one other change in page_io.c
where count_mthp_stat() is called by count_swpout_vm_event(). I would
appreciate it if you can review this. Thanks!
Hopefully this should resolve the kernel robot build errors.

Changes since v2:
=================
1) Gathered usemem data using SSD as the backing swap device for zswap,
as suggested by Ying Huang. Ying, I would appreciate it if you can
review the latest data. Thanks!
2) Generated the base commit info in the patches to attempt to address
the kernel test robot build errors.
3) No code changes to the individual patches themselves.

Changes since RFC v1:
=====================

1) Use sysfs for zswpout mTHP stats, as per Barry Song's suggestion.
Thanks Barry!
2) Addressed some of the code review comments that Nhat Pham provided in
Ryan's initial RFC [1]:
- Added a comment about the cgroup zswap limit checks occuring once per
folio at the beginning of zswap_store().
Nhat, Ryan, please do let me know if the comments convey the summary
from the RFC discussion. Thanks!
- Posted data on running the cgroup suite's zswap kselftest.
3) Rebased to v6.11-rc3.
4) Gathered performance data with usemem and the rebased patch-series.

Regression Testing:
===================
I ran vm-scalability usemem 70 processes without mTHP, i.e., only 4K
folios with mm-unstable and with this patch-series. The main goal was
to make sure that there is no functional or performance regression
wrt the earlier zswap behavior for 4K folios,
CONFIG_ZSWAP_STORE_THP_DEFAULT_ON is not set, and zswap_store() of 4K
pages goes through the newly added code path [zswap_store(),
zswap_store_page()].

The data indicates there is no regression.

------------------------------------------------------------------------------
mm-unstable 8-28-2024 zswap-mTHP v6
CONFIG_ZSWAP_STORE_THP_DEFAULT_ON
is not set
------------------------------------------------------------------------------
ZSWAP compressor zstd deflate- zstd deflate-
iaa iaa
------------------------------------------------------------------------------
Throughput (KB/s) 110,775 113,010 111,550 121,937
sys time (sec) 1,141.72 954.87 1,131.95 828.47
memcg_high 140,500 153,737 139,772 134,129
memcg_swap_high 0 0 0 0
memcg_swap_fail 0 0 0 0
pswpin 0 0 0 0
pswpout 0 0 0 0
zswpin 675 690 682 684
zswpout 9,552,298 10,603,271 9,566,392 9,267,213
thp_swpout 0 0 0 0
thp_swpout_ 0 0 0 0
fallback
pgmajfault 3,453 3,468 3,841 3,487
ZSWPOUT-64kB-mTHP n/a n/a 0 0
SWPOUT-64kB-mTHP 0 0 0 0
------------------------------------------------------------------------------

Performance Testing:
====================
Testing of this patch-series was done with the v6.11-rc3 mainline, without
and with this patch-series, on an Intel Sapphire Rapids server,
dual-socket 56 cores per socket, 4 IAA devices per socket.

The system has 503 GiB RAM, with 176GiB ZRAM (35% of available RAM) as the
backing swap device for ZSWAP. zstd is configured as the ZRAM compressor.
Core frequency was fixed at 2500MHz.

The vm-scalability "usemem" test was run in a cgroup whose memory.high
was fixed at 40G. The is no swap limit set for the cgroup. Following a
similar methodology as in Ryan Roberts' "Swap-out mTHP without splitting"
series [2], 70 usemem processes were run, each allocating and writing 1G of
memory:

usemem --init-time -w -O -n 70 1g

The vm/sysfs mTHP stats included with the performance data provide details
on the swapout activity to ZSWAP/swap.

Other kernel configuration parameters:

ZSWAP Compressors : zstd, deflate-iaa
ZSWAP Allocator : zsmalloc
SWAP page-cluster : 2

In the experiments where "deflate-iaa" is used as the ZSWAP compressor,
IAA "compression verification" is enabled. Hence each IAA compression
will be decompressed internally by the "iaa_crypto" driver, the crc-s
returned by the hardware will be compared and errors reported in case of
mismatches. Thus "deflate-iaa" helps ensure better data integrity as
compared to the software compressors.

Throughput is derived by averaging the individual 70 processes' throughputs
reported by usemem. sys time is measured with perf. All data points are
averaged across 3 runs.

Case 1: Baseline with CONFIG_THP_SWAP turned off, and mTHP is split in reclaim.
===============================================================================

In this scenario, the "before" is CONFIG_THP_SWAP set to off, that results in
64K/2M (m)THP to be split, and only 4K folios processed by zswap.

The "after" is CONFIG_THP_SWAP set to on, and this patch-series, that results
in 64K/2M (m)THP to not be split, and processed by zswap.

64KB mTHP (cgroup memory.high set to 40G):
==========================================

-------------------------------------------------------------------------------
v6.11-rc3 mainline zswap-mTHP Change wrt
Baseline Baseline
CONFIG_THP_SWAP=N CONFIG_THP_SWAP=Y
-------------------------------------------------------------------------------
ZSWAP compressor zstd deflate- zstd deflate- zstd deflate-
iaa iaa iaa
-------------------------------------------------------------------------------
Throughput (KB/s) 136,113 140,044 140,363 151,938 3% 8%
sys time (sec) 986.78 951.95 954.85 735.47 3% 23%
memcg_high 124,183 127,513 138,651 133,884
memcg_swap_high 0 0 0 0
memcg_swap_fail 619,020 751,099 0 0
pswpin 0 0 0 0
pswpout 0 0 0 0
zswpin 656 569 624 639
zswpout 9,413,603 11,284,812 9,453,761 9,385,910
thp_swpout 0 0 0 0
thp_swpout_ 0 0 0 0
fallback
pgmajfault 3,470 3,382 4,633 3,611
ZSWPOUT-64kB n/a n/a 590,768 586,521
SWPOUT-64kB 0 0 0 0
-------------------------------------------------------------------------------

2MB PMD-THP/2048K mTHP (cgroup memory.high set to 40G):
=======================================================

------------------------------------------------------------------------------
v6.11-rc3 mainline zswap-mTHP Change wrt
Baseline Baseline
CONFIG_THP_SWAP=N CONFIG_THP_SWAP=Y
------------------------------------------------------------------------------
ZSWAP compressor zstd deflate- zstd deflate- zstd deflate-
iaa iaa iaa
------------------------------------------------------------------------------
Throughput (KB/s) 164,220 172,523 165,005 174,536 0.5% 1%
sys time (sec) 855.76 686.94 801.72 676.65 6% 1%
memcg_high 14,628 16,247 14,951 16,096
memcg_swap_high 0 0 0 0
memcg_swap_fail 18,698 21,114 0 0
pswpin 0 0 0 0
pswpout 0 0 0 0
zswpin 663 665 5,333 781
zswpout 8,419,458 8,992,065 8,546,895 9,355,760
thp_swpout 0 0 0 0
thp_swpout_ 18,697 21,113 0 0
fallback
pgmajfault 3,439 3,496 8,139 3,582
ZSWPOUT-2048kB n/a n/a 16,684 18,270
SWPOUT-2048kB 0 0 0 0
-----------------------------------------------------------------------------

We see improvements overall in throughput and sys time for zstd and
deflate-iaa, when comparing before (THP_SWAP=N) vs. after (THP_SWAP=Y).

Case 2: Baseline with CONFIG_THP_SWAP enabled.
==============================================

In this scenario, the "before" represents zswap rejecting mTHP, and the mTHP
being stored by the backing swap device.

The "after" represents data with this patch-series, that results in 64K/2M
(m)THP being processed by zswap.

64KB mTHP (cgroup memory.high set to 40G):
==========================================

------------------------------------------------------------------------------
v6.11-rc3 mainline zswap-mTHP Change wrt
Baseline Baseline
------------------------------------------------------------------------------
ZSWAP compressor zstd deflate- zstd deflate- zstd deflate-
iaa iaa iaa
------------------------------------------------------------------------------
Throughput (KB/s) 161,496 156,343 140,363 151,938 -13% -3%
sys time (sec) 771.68 802.08 954.85 735.47 -24% 8%
memcg_high 111,223 110,889 138,651 133,884
memcg_swap_high 0 0 0 0
memcg_swap_fail 0 0 0 0
pswpin 16 16 0 0
pswpout 7,471,472 7,527,963 0 0
zswpin 635 605 624 639
zswpout 1,509 1,478 9,453,761 9,385,910
thp_swpout 0 0 0 0
thp_swpout_ 0 0 0 0
fallback
pgmajfault 3,616 3,430 4,633 3,611
ZSWPOUT-64kB n/a n/a 590,768 586,521
SWPOUT-64kB 466,967 470,498 0 0
------------------------------------------------------------------------------

2MB PMD-THP/2048K mTHP (cgroup memory.high set to 40G):
=======================================================

------------------------------------------------------------------------------
v6.11-rc3 mainline zswap-mTHP Change wrt
Baseline Baseline
------------------------------------------------------------------------------
ZSWAP compressor zstd deflate- zstd deflate- zstd deflate-
iaa iaa iaa
------------------------------------------------------------------------------
Throughput (KB/s) 192,164 194,643 165,005 174,536 -14% -10%
sys time (sec) 823.55 830.42 801.72 676.65 3% 19%
memcg_high 16,054 15,936 14,951 16,096
memcg_swap_high 0 0 0 0
memcg_swap_fail 0 0 0 0
pswpin 0 0 0 0
pswpout 8,629,248 8,628,907 0 0
zswpin 560 645 5,333 781
zswpout 1,416 1,503 8,546,895 9,355,760
thp_swpout 16,854 16,853 0 0
thp_swpout_ 0 0 0 0
fallback
pgmajfault 3,341 3,574 8,139 3,582
ZSWPOUT-2048kB n/a n/a 16,684 18,270
SWPOUT-2048kB 16,854 16,853 0 0
------------------------------------------------------------------------------

In the "Before" scenario, when zswap does not store mTHP, only allocations
count towards the cgroup memory limit. However, in the "After" scenario,
with the introduction of zswap_store() mTHP, both, allocations as well as
the zswap compressed pool usage from all 70 processes are counted towards
the memory limit. As a result, we see higher swapout activity in the
"After" data. Hence, more time is spent doing reclaim as the zswap cgroup
charge leads to more frequent memory.high breaches.

This causes degradation in throughput and sys time with zswap mTHP, more so
in case of zstd than deflate-iaa. Compress latency could play a part in
this - when there is more swapout activity happening, a slower compressor
would cause allocations to stall for any/all of the 70 processes.

In my opinion, even though the test set up does not provide an accurate
way for a direct before/after comparison (because of zswap usage being
counted in cgroup, hence towards the memory.high), it still seems
reasonable for zswap_store to support (m)THP, so that further performance
improvements can be implemented.

One of the ideas that has shown promise in our experiments is to improve
ZSWAP mTHP store performance using batching. With IAA compress/decompress
batching used in ZSWAP, we are able to demonstrate significant
performance improvements and memory savings with IAA in scalability
experiments, as compared to software compressors. We hope to submit
this work as subsequent RFCs.

I would greatly appreciate your code review comments and suggestions!

Thanks,
Kanchana

[2] https://lore.kernel.org/linux-mm/20240408183946.2991168-1-ryan.roberts@arm.com/

Kanchana P Sridhar (3):
mm: Define obj_cgroup_get() if CONFIG_MEMCG is not defined.
mm: zswap: zswap_store() extended to handle mTHP folios.
mm: swap: Count successful mTHP ZSWAP stores in sysfs mTHP zswpout
stats.

include/linux/huge_mm.h | 1 +
include/linux/memcontrol.h | 4 +
mm/Kconfig | 8 ++
mm/huge_memory.c | 3 +
mm/page_io.c | 3 +-
mm/zswap.c | 243 +++++++++++++++++++++++++++----------
6 files changed, 200 insertions(+), 62 deletions(-)

base-commit: 9287e4adbc6ab8fa04d25eb82e097fed877a4642
--
2.27.0