When zswap writeback is enabled and it fails compressing a given page,
the page is swapped out to the backing swap device. This behavior
breaks the zswap's writeback LRU order, and hence users can experience
unexpected latency spikes. If the page is compressed without failure,
but results in a size of PAGE_SIZE, the LRU order is kept, but the
decompression overhead for loading the page back on the later access is
unnecessary.
Keep the LRU order and optimize unnecessary decompression overheads in
the cases, by storing the original content in zpool as-is. The length
field of zswap_entry will be set appropriately, as PAGE_SIZE, Hence
whether it is saved as-is or not (whether decompression is unnecessary)
is identified by 'zswap_entry->length == PAGE_SIZE'.
So this change is not increasing per zswap entry metadata overhead. But
as the number of incompressible pages increases, total zswap metadata
overhead is proportionally increased. The overhead should not be
problematic in usual cases, since the zswap metadata for single zswap
entry is much smaller than PAGE_SIZE, and in common zswap use cases
there should be a sufficient amount of compressible pages. Also it can
be mitigated by the zswap writeback.
When a severe memory pressure comes from memcg's memory.high, storing
incompressible pages as-is may result in reducing accounted memory
footprint slower, since the footprint will be reduced only after the
zswap writeback kicks in. This can incur higher penalty_jiffies and
degrade the performance. Arguably this is just a wrong setup, but we
don't want to introduce unnecessary surprises. Add a parameter, namely
'save_incompressible_pages', to turn the feature on/off as users want.
It is turned off by default.
When the writeback is disabled, the additional overhead could be
problematic. For the case, keep the current behavior that just returns
the failure and let swap_writeout() put the page back to the active LRU
list in the case. It is known to be suboptimal when the incompressible
pages are cold, since the incompressible pages will continuously be
tried to be zswapped out, and burn CPU cycles for compression attempts
that will anyway fails. One imaginable solution for the problem is
reusing the swapped-out page and its struct page to store in the zswap
pool. But that's out of the scope of this patch.
Tests
-----
I tested this patch using a simple self-written microbenchmark that is
available at GitHub[1]. You can reproduce the test I did by executing
run_tests.sh of the repo on your system. Note that the repo's
documentation is not good as of this writing, so you may need to read
and use the code.
The basic test scenario is simple. Run a test program making artificial
accesses to memory having artificial content under memory.high-set
memory limit and measure how many accesses were made in given time.
The test program repeatedly and randomly access three anonymous memory
regions. The regions are all 500 MiB size, and accessed in the same
probability. Two of those are filled up with a simple content that can
easily be compressed, while the remaining one is filled up with a
content that read from /dev/urandom, which is easy to fail at
compressing to <PAGE_SIZE size. The program runs for two minutes and
prints out the number of accesses made every five seconds.
The test script runs the program under below seven configurations.
- 0: memory.high is set to 2 GiB, zswap is disabled.
- 1-1: memory.high is set to 1350 MiB, zswap is disabled.
- 1-2: Same to 1-1, but zswap is enabled.
- 1-3: Same to 1-2, but save_incompressible_pages is turned on.
- 2-1: memory.high is set to 1200 MiB, zswap is disabled.
- 2-2: Same to 2-1, but zswap is enabled.
- 2-3: Same to 2-2, but save_incompressible_pages is turned on.
For all zswap enabled case, zswap shrinker is enabled.
Configuration '0' is for showing the original memory performance.
Configurations 1-1, 1-2 and 1-3 are for showing the performance of swap,
zswap, and this patch under a level of memory pressure (~10% of working
set).
Configurations 2-1, 2-2 and 2-3 are similar to 1-1, 1-2 and 1-3 but to
show those under a severe level of memory pressure (~20% of the working
set).
Because the per-5 seconds performance is not very reliable, I measured
the average of that for the last one minute period of the test program
run. I also measured a few vmstat counters including zswpin, zswpout,
zswpwb, pswpin and pswpout during the test runs.
The measurement results are as below. To save space, I show performance
numbers that are normalized to that of the configuration '0' (no memory
pressure), only. The averaged accesses per 5 seconds of configuration
'0' was 36493417.75.
config 0 1-1 1-2 1-3 2-1 2-2 2-3
perf_normalized 1.0000 0.0057 0.0235 0.0367 0.0031 0.0122 0.0077
perf_stdev_ratio 0.0582 0.0652 0.0167 0.0346 0.0404 0.0145 0.0613
zswpin 0 0 3548424 1999335 0 2912972 1612517
zswpout 0 0 3588817 2361689 0 2996588 2029884
zswpwb 0 0 10214 340270 0 34625 382117
pswpin 0 485806 772038 340967 540476 874909 790418
pswpout 0 649543 144773 340270 692666 275178 382117
'perf_normalized' is the performance metric, normalized to that of
configuration '0' (no pressure). 'perf_stdev_ratio' is the standard
deviation of the averaged data points, as a ratio to the averaged metric
value. For example, configuration '0' performance was showing 5.8%
stdev. Configurations 1-1 and 1-3 were having about 6.5% and 6.1%
stdev. Also the results were highly variable between multiple runs. So
this result is not very stable but just showing ball park figures.
Please keep this in your mind when reading these results.
Under about 10% of working set memory pressure, the performance was
dropped to about 0.57% of no-pressure one, when the normal swap is used
(1-1). Actually ~10% working set pressure is not a mild one, at least
on this test setup.
By turning zswap on (1-2), the performance was improved about 4x,
resulting in about 2.35% of no-pressure one. Because of the
incompressible pages in the third memory region, a significant amount of
(non-zswap) swap I/O operations were made, though.
By enabling the incompressible pages handling feature that is introduced
by this patch (1-3), about 56% performance improvement was made,
resulting in about 3.67% of no-pressure one. Reduced pswpin of 1-3
compared to 1-2 let us see where this improvement came from.
Under about 20% of working set memory pressure, which could be extreme,
the performance drops down to 0.31% of no-pressure one when only the
normal swap is used (2-1). Enabling zswap significantly improves it, up
to 1.22%, though again showing a significant number of (non-zswap) swap
I/O due to incompressible pages.
Enabling the incompressible pages handling feature of this patch (2-3)
didn't reduce non-zswap swap I/O, because the memory pressure is too
severe to let nearly all zswap pages including the incompressible pages
written back by zswap shrinker. And because the memory usage is not
dropped as soon as incompressible pages are swapped out but only after
those are written back by shrinker, memory.high apparently applied more
penalty_jiffies. As a result, the performance became even worse than
2-2 about 36.88%, resulting in 0.07% of the no-pressure one.
20% of working set memory pressure is pretty extreme, but anyway the
incompressible pages handling feature could make it worse in certain
setups. Hence add the parameter for turning the feature on/off as
needed, and disable it by default.
Related Works
-------------
This is not an entirely new attempt. Nhat Pham and Takero Funaki tried
very similar approaches in October 2023[2] and April 2024[3],
respectively. The two approaches didn't get merged mainly due to the
metadata overhead concern. I described why I think that shouldn't be a
problem for this change, which is automatically disabled when writeback
is disabled, at the beginning of this changelog.
This patch is not particularly different from those, and actually built
upon those. I wrote this from scratch again, though. Hence adding
Suggested-by tags for them. Actually Nhat first suggested this to me
offlist.
[1] https://github.com/sjp38/eval_zswap/blob/master/run.sh
[2] https://lore.kernel.org/20231017003519.1426574-3-nphamcs@xxxxxxxxx
[3] https://lore.kernel.org/20240706022523.1104080-6-flintglass@xxxxxxxxx
Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
Suggested-by: Takero Funaki <flintglass@xxxxxxxxx>
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
---
Changes from RFC v1
(https://lore.kernel.org/20250730234059.4603-1-sj@xxxxxxxxxx)
- Consider PAGE_SIZE-resulting compression successes as failures.
- Use zpool for storing incompressible pages.
- Test with zswap shrinker enabled.
- Wordsmith changelog and comments.
- Add documentation of save_incompressible_pages parameter.
Documentation/admin-guide/mm/zswap.rst | 9 +++++
mm/zswap.c | 53 +++++++++++++++++++++++++-
2 files changed, 61 insertions(+), 1 deletion(-)
diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
index c2806d051b92..20eae0734491 100644
--- a/Documentation/admin-guide/mm/zswap.rst
+++ b/Documentation/admin-guide/mm/zswap.rst
@@ -142,6 +142,15 @@ User can enable it as follows::
This can be enabled at the boot time if ``CONFIG_ZSWAP_SHRINKER_DEFAULT_ON`` is
selected.
+If a page cannot be compressed into a size smaller than PAGE_SIZE, it can be
+beneficial to save the content as is without compression, to keep the LRU
+order. Users can enable this behavior, as follows::
+
+ echo Y > /sys/module/zswap/parameters/save_incompressible_pages
+
+This is disabled by default, and doesn't change behavior of zswap writeback
+disabled case.
+
A debugfs interface is provided for various statistic about pool size, number
of pages stored, same-value filled pages and various counters for the reasons
pages are rejected.
diff --git a/mm/zswap.c b/mm/zswap.c
index 7e02c760955f..6e196c9a4dba 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -129,6 +129,11 @@ static bool zswap_shrinker_enabled = IS_ENABLED(
CONFIG_ZSWAP_SHRINKER_DEFAULT_ON);
module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
+/* Enable/disable incompressible pages storing */
+static bool zswap_save_incompressible_pages;
+module_param_named(save_incompressible_pages, zswap_save_incompressible_pages,
+ bool, 0644);
+
bool zswap_is_enabled(void)
{
return zswap_enabled;
@@ -937,6 +942,29 @@ static void acomp_ctx_put_unlock(struct crypto_acomp_ctx *acomp_ctx)
mutex_unlock(&acomp_ctx->mutex);
}
+/*
+ * Determine whether to save given page as-is.
+ *
+ * If a page cannot be compressed into a size smaller than PAGE_SIZE, it can be
+ * beneficial to saving the content as is without compression, to keep the LRU
+ * order. This can increase memory overhead from metadata, but in common zswap
+ * use cases where there are sufficient amount of compressible pages, the
+ * overhead should be not critical, and can be mitigated by the writeback.
+ * Also, the decompression overhead is optimized.
+ *
+ * When the writeback is disabled, however, the additional overhead could be
+ * problematic. For the case, just return the failure. swap_writeout() will
+ * put the page back to the active LRU list in the case.
+ */
+static bool zswap_save_as_is(int comp_ret, unsigned int dlen,
+ struct page *page)
+{
+ return zswap_save_incompressible_pages &&
+ (comp_ret || dlen == PAGE_SIZE) &&
+ mem_cgroup_zswap_writeback_enabled(
+ folio_memcg(page_folio(page)));
+}
+
static bool zswap_compress(struct page *page, struct zswap_entry *entry,
struct zswap_pool *pool)
{
@@ -976,8 +1004,13 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
*/
comp_ret = crypto_wait_req(crypto_acomp_compress(acomp_ctx->req), &acomp_ctx->wait);
dlen = acomp_ctx->req->dlen;
- if (comp_ret)
+ if (zswap_save_as_is(comp_ret, dlen, page)) {
+ comp_ret = 0;
+ dlen = PAGE_SIZE;
+ memcpy_from_page(dst, page, 0, dlen);
+ } else if (comp_ret) {
goto unlock;
+ }
zpool = pool->zpool;
gfp = GFP_NOWAIT | __GFP_NORETRY | __GFP_HIGHMEM | __GFP_MOVABLE;
@@ -1001,6 +1034,17 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
return comp_ret == 0 && alloc_ret == 0;
}
+/*
+ * If save_incompressible_pages is set and writeback is enabled, incompressible
+ * pages are saved as is without compression. For more details, refer to the
+ * comments of zswap_save_as_is().
+ */
+static bool zswap_saved_as_is(struct zswap_entry *entry, struct folio *folio)
+{
+ return entry->length == PAGE_SIZE && zswap_save_incompressible_pages &&
+ mem_cgroup_zswap_writeback_enabled(folio_memcg(folio));
+}
+
static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
{
struct zpool *zpool = entry->pool->zpool;
@@ -1012,6 +1056,13 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
acomp_ctx = acomp_ctx_get_cpu_lock(entry->pool);
obj = zpool_obj_read_begin(zpool, entry->handle, acomp_ctx->buffer);
+ if (zswap_saved_as_is(entry, folio)) {
+ memcpy_to_folio(folio, 0, obj, entry->length);
+ zpool_obj_read_end(zpool, entry->handle, obj);
+ acomp_ctx_put_unlock(acomp_ctx);
+ return true;
+ }
+
/*
* zpool_obj_read_begin() might return a kmap address of highmem when
* acomp_ctx->buffer is not used. However, sg_init_one() does not
base-commit: d19f69751d55ef3883569c119d4b2ea3d6a0e39f
--
2.39.5
