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Diffstat (limited to '')
| -rw-r--r-- | Documentation/admin-guide/mm/zswap.rst (renamed from Documentation/vm/zswap.rst) | 50 |
1 files changed, 35 insertions, 15 deletions
diff --git a/Documentation/vm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst index 61f6185188cd..6e6f7b0d6562 100644 --- a/Documentation/vm/zswap.rst +++ b/Documentation/admin-guide/mm/zswap.rst @@ -10,7 +10,7 @@ Overview Zswap is a lightweight compressed cache for swap pages. It takes pages that are in the process of being swapped out and attempts to compress them into a dynamically allocated RAM-based memory pool. zswap basically trades CPU cycles -for potentially reduced swap I/O. This trade-off can also result in a +for potentially reduced swap I/O. This trade-off can also result in a significant performance improvement if reads from the compressed cache are faster than reads from a swap device. @@ -26,7 +26,7 @@ faster than reads from a swap device. performance impact of swapping. * Overcommitted guests that share a common I/O resource can dramatically reduce their swap I/O pressure, avoiding heavy handed I/O - throttling by the hypervisor. This allows more work to get done with less + throttling by the hypervisor. This allows more work to get done with less impact to the guest workload and guests sharing the I/O subsystem * Users with SSDs as swap devices can extend the life of the device by drastically reducing life-shortening writes. @@ -35,9 +35,11 @@ Zswap evicts pages from compressed cache on an LRU basis to the backing swap device when the compressed pool reaches its size limit. This requirement had been identified in prior community discussions. -Zswap is disabled by default but can be enabled at boot time by setting -the ``enabled`` attribute to 1 at boot time. ie: ``zswap.enabled=1``. Zswap -can also be enabled and disabled at runtime using the sysfs interface. +Whether Zswap is enabled at the boot time depends on whether +the ``CONFIG_ZSWAP_DEFAULT_ON`` Kconfig option is enabled or not. +This setting can then be overridden by providing the kernel command line +``zswap.enabled=`` option, for example ``zswap.enabled=0``. +Zswap can also be enabled and disabled at runtime using the sysfs interface. An example command to enable zswap at runtime, assuming sysfs is mounted at ``/sys``, is:: @@ -64,9 +66,10 @@ allocation in zpool is not directly accessible by address. Rather, a handle is returned by the allocation routine and that handle must be mapped before being accessed. The compressed memory pool grows on demand and shrinks as compressed pages are freed. The pool is not preallocated. By default, a zpool -of type zbud is created, but it can be selected at boot time by -setting the ``zpool`` attribute, e.g. ``zswap.zpool=zbud``. It can -also be changed at runtime using the sysfs ``zpool`` attribute, e.g.:: +of type selected in ``CONFIG_ZSWAP_ZPOOL_DEFAULT`` Kconfig option is created, +but it can be overridden at boot time by setting the ``zpool`` attribute, +e.g. ``zswap.zpool=zbud``. It can also be changed at runtime using the sysfs +``zpool`` attribute, e.g.:: echo zbud > /sys/module/zswap/parameters/zpool @@ -97,8 +100,9 @@ controlled policy: * max_pool_percent - The maximum percentage of memory that the compressed pool can occupy. -The default compressor is lzo, but it can be selected at boot time by -setting the ``compressor`` attribute, e.g. ``zswap.compressor=lzo``. +The default compressor is selected in ``CONFIG_ZSWAP_COMPRESSOR_DEFAULT`` +Kconfig option, but it can be overridden at boot time by setting the +``compressor`` attribute, e.g. ``zswap.compressor=lzo``. It can also be changed at runtime using the sysfs "compressor" attribute, e.g.:: @@ -126,9 +130,25 @@ attribute, e.g.:: echo 1 > /sys/module/zswap/parameters/same_filled_pages_enabled When zswap same-filled page identification is disabled at runtime, it will stop -checking for the same-value filled pages during store operation. However, the -existing pages which are marked as same-value filled pages remain stored -unchanged in zswap until they are either loaded or invalidated. +checking for the same-value filled pages during store operation. +In other words, every page will be then considered non-same-value filled. +However, the existing pages which are marked as same-value filled pages remain +stored unchanged in zswap until they are either loaded or invalidated. + +In some circumstances it might be advantageous to make use of just the zswap +ability to efficiently store same-filled pages without enabling the whole +compressed page storage. +In this case the handling of non-same-value pages by zswap (enabled by default) +can be disabled by setting the ``non_same_filled_pages_enabled`` attribute +to 0, e.g. ``zswap.non_same_filled_pages_enabled=0``. +It can also be enabled and disabled at runtime using the sysfs +``non_same_filled_pages_enabled`` attribute, e.g.:: + + echo 1 > /sys/module/zswap/parameters/non_same_filled_pages_enabled + +Disabling both ``zswap.same_filled_pages_enabled`` and +``zswap.non_same_filled_pages_enabled`` effectively disables accepting any new +pages by zswap. To prevent zswap from shrinking pool when zswap is full and there's a high pressure on swap (this will result in flipping pages in and out zswap pool @@ -136,10 +156,10 @@ without any real benefit but with a performance drop for the system), a special parameter has been introduced to implement a sort of hysteresis to refuse taking pages into zswap pool until it has sufficient space if the limit has been hit. To set the threshold at which zswap would start accepting pages -again after it became full, use the sysfs ``accept_threhsold_percent`` +again after it became full, use the sysfs ``accept_threshold_percent`` attribute, e. g.:: - echo 80 > /sys/module/zswap/parameters/accept_threhsold_percent + echo 80 > /sys/module/zswap/parameters/accept_threshold_percent Setting this parameter to 100 will disable the hysteresis. |