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6f34b4126b
Currently, shrink_memcg() writes back at most one entry per-node during
its traversal. This makes shrink_worker() inefficient, as it must
repeatedly re-enter shrink_memcg() to make any substantial progress.
Under high memory pressure, this can cause the writeback speed to be too
slow to keep up with refaults, leading to zswap store failures and forcing
pages to skip zswap and go directly to disk, which results in an LRU
inversion.
To address this, extend the per-node scan budget in shrink_memcg() from a
single entry to up to SWAP_CLUSTER_MAX pages, enabling batch writeback for
both the shrink_worker() and zswap_store() paths.
Test Setup:
- Total memory: 32 GB, 1 NUMA node.
- zswap settings: accept_threshold_percent=50, shrinker_enabled=N.
Test Case 1:
Set max_pool_percent=1, allocate 512MB of anonymous pages, and fill them
with random data (to avoid compression). Then, use cgroup memory.reclaim
to force a large amount of anonymous pages into zswap. At an interval of
2ms, allocate a 4K anonymous page where the first 4 bytes are random numbers
and the rest are zeros, and then trigger reclamation of this 4K page through
cgroup memory.reclaim. When the pool threshold is reached, shrink_memcg()
will be triggered.
The test data after running for 120s is as follows:
Baseline Patched
shrink_worker wakeups 5,363 169
shrink_memcg calls 11,373,201 350,703
written_back pages 40,212 40,241
zswap_store calls 161,190 163,753
store succeeded (ret=1) 102,743 117,183
store rejected (ret=0) 58,447 46,570
store reject rate ~36% ~28%
pool_limit_hit delta 55,826 33,760
pswpout 98,659 86,811
pswpin 2 0
Test Case 2:
We evaluated the following two sub-configurations using stress-ng inside
a cgroup capped at memory.max=1G for 120 seconds:
Test Case 2a (max_pool_percent=1): Continuously triggers the global
zswap pool limit, thereby waking up shrink_worker() to perform asynchronous
shrinking.
Test Case 2b (zswap.max=320M, max_pool_percent=50): Continuously triggers
the cgroup's zswap.max limit, thereby invoking synchronous shrinking.
Command executed for both setups:
bash -c 'echo $$ > /sys/fs/cgroup/zswaptest/cgroup.procs ; \
exec stress-ng --vm 4 --vm-bytes 4G --vm-keep --vm-method rand-set -t \
120s -q'
Test Case 2a (max_pool_percent=1):
Baseline Patched
shrink_worker wakeups 5,640 1,308
shrink_memcg calls 8,481,500 3,140,972
written_back pages 260 468,216
zswap_store calls 2,742,756 2,011,269
store succeeded (ret=1) 934,640 947,988
store rejected (ret=0) 1,808,116 1,063,281
store reject rate ~66% ~52%
pool_limit_hit delta 1,181,310 196,882
pswpout 1,808,376 1,531,497
pswpin 4,288,497 3,635,365
Test Case 2b (zswap.max=320M, max_pool_percent=50):
Baseline Patched
shrink_worker wakeups 0 0
shrink_memcg calls 687,608 54,002
written_back pages 639,176 846,663
zswap_store calls 1,224,222 1,228,548
store succeeded (ret=1) 992,816 1,208,123
store rejected (ret=0) 231,431 20,425
store reject rate ~19% ~2%
pool_limit_hit delta 0 0
pswpout 870,745 867,360
pswpin 1,707,823 1,216,814
Under identical workloads and runtimes, batched zswap shrinking exhibits a
significant reduction in both shrink_worker() wakeups and shrink_memcg()
calls. Furthermore, the sharp drop in both pswpin and zswap_store()
rejections demonstrates that batching zswap shrink operations effectively
mitigates zswap_store() failures caused by hitting the pool limit. This
significantly prevents pages from bypassing zswap and falling back
directly to disk, thereby reducing LRU inversion.
Link: https://lore.kernel.org/20260806070943.95542-3-jiahao.kernel@gmail.com
Signed-off-by: Hao Jia <jiahao1@lixiang.com>
Suggested-by: Yosry Ahmed <yosry@kernel.org>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Yosry Ahmed <yosry@kernel.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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