zswap is a kernel feature that provides a compressed RAM cache for swap pages. Pages which would otherwise be swapped out to disk are instead compressed and stored into a memory pool in RAM. Once the pool is full or the RAM is exhausted, the least recently used (LRU) page is decompressed and written to disk, as if it had not been intercepted. After the page has been decompressed into the swap cache, the compressed version in the pool can be freed.
All officially supported kernels have zswap enabled by default. This can be verified with
zgrep CONFIG_ZSWAP_DEFAULT_ON /proc/config.gz.
To disable zswap at runtime, execute the following command:
# echo 0 > /sys/module/zswap/parameters/enabled
To disable zswap permanently, add
zswap.enabled=0 to your kernel parameters.
zswap has several customizable parameters. The live settings can be displayed using:
$ grep -r . /sys/module/zswap/parameters/
/sys/module/zswap/parameters/same_filled_pages_enabled:Y /sys/module/zswap/parameters/enabled:Y /sys/module/zswap/parameters/max_pool_percent:20 /sys/module/zswap/parameters/compressor:zstd /sys/module/zswap/parameters/non_same_filled_pages_enabled:Y /sys/module/zswap/parameters/zpool:zsmalloc /sys/module/zswap/parameters/exclusive_loads:N /sys/module/zswap/parameters/accept_threshold_percent:90
See the zswap documentation for the description of the different parameters.
For more information about
exclusive_loads parameter (which is not currently in zswap documentation), see this commentary in linux kernel source code.
The boot time load message showing the initial configuration can be retrieved with:
# dmesg | grep zswap:
[ 0.317569] zswap: loaded using pool zstd/zsmalloc
Each setting can be changed at runtime via the sysfs interface. For example, to change the
# echo lz4 > /sys/module/zswap/parameters/compressor
Using kernel boot parameters
To persist the parameter change, the corresponding option, for example
zswap.compressor=lz4, must be added to the kernel boot parameter. Therefore to set permanently all the above settings, the following kernel parameters must be added:
zswap.enabled=1 zswap.compressor=lz4 zswap.max_pool_percent=20 zswap.zpool=z3fold
When changing the compression algorithm via boot parameter, one needs to ensure the corresponding compression module is loaded early during boot (refer to #Compression algorithm).
Maximum pool size
The memory pool is not preallocated, it is allowed to grow up to a certain limit in percentage of the total memory available, by default up to 20% of the total RAM. Once this threshold is reached, pages are evicted from the pool into the swap device. The maximum compressed pool size is controlled with the parameter
Compressed memory pool allocator
The zpool parameter controls the management of the compressed memory pool.
With the zbud data allocator, 2 compressed objects are stored into 1 page which limits the compression ratio to 2 or less.
z3fold allocator allows up to 3 compressed objects by page. The compression ratio with z3fold typically averages 2.7 while it is 1.7 for zbud.
A zpool of type z3fold is created by default. Use the kernel parameter
zswap.zpool to select another allocator at boot time. The data allocator can also be changed at a later stage via the sysfs interface.
In later kernels (after 6.3.arch1-1+) zsmalloc allocator was added. It is supposed to work well under low memory conditions and it saves more memory.
For page compression, zswap uses compressor modules provided by the kernel's cryptographic API. In official kernels the zstd compression algorithm is used by default but this can be changed with
zswap.compressor= at boot time. Other options include deflate, lzo, 842, lz4 and lz4hc.
There is no issue changing the compression at runtime using sysfs but zswap starts in this case with zstd and switches at a later stage to the defined algorithm. To start zswap with another algorithm straight away, this must be set via the kernel boot parameters and the corresponding module must be loaded early by the kernel. This can be achieved by following these steps:
- Add the modules required for the chosen compressor to the mkinitcpio#MODULES array.
- Regenerate the initramfs.
- Set the compression algorithm using the
On next boot, see #Current parameters to check if zswap now uses the requested compressor.
To see zswap statistics you can run this:
# grep -r . /sys/kernel/debug/zswap
duplicate_entry:0 pool_limit_hit:13404213 pool_total_size:6184960 (pool size in total in pages) reject_alloc_fail:5 reject_compress_poor:0 reject_kmemcache_fail:0 reject_reclaim_fail:13404213 stored_pages:4251 (pool size after compression) written_back_pages:0
- zswap: How to determine whether it is compressing swap pages?.
- IBM Support Article "New Linux zswap compression functionality" (benchmarks images do not load).
- Ask Ubuntu: zram vs. zswap vs. zcache. (zcache is deprecated)
- Arch Linux forum thread.
- LWN.net technical article by the main developer of zswap.