| ===== |
| zswap |
| ===== |
| |
| 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 |
| significant performance improvement if reads from the compressed cache are |
| faster than reads from a swap device. |
| |
| Some potential benefits: |
| |
| * Desktop/laptop users with limited RAM capacities can mitigate the |
| 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 |
| 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. |
| |
| 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. |
| |
| 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:: |
| |
| echo 1 > /sys/module/zswap/parameters/enabled |
| |
| When zswap is disabled at runtime it will stop storing pages that are |
| being swapped out. However, it will _not_ immediately write out or fault |
| back into memory all of the pages stored in the compressed pool. The |
| pages stored in zswap will remain in the compressed pool until they are |
| either invalidated or faulted back into memory. In order to force all |
| pages out of the compressed pool, a swapoff on the swap device(s) will |
| fault back into memory all swapped out pages, including those in the |
| compressed pool. |
| |
| Design |
| ====== |
| |
| Zswap receives pages for compression from the swap subsystem and is able to |
| evict pages from its own compressed pool on an LRU basis and write them back to |
| the backing swap device in the case that the compressed pool is full. |
| |
| Zswap makes use of zpool for the managing the compressed memory pool. Each |
| 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 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 |
| |
| The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which |
| means the compression ratio will always be 2:1 or worse (because of half-full |
| zbud pages). The zsmalloc type zpool has a more complex compressed page |
| storage method, and it can achieve greater storage densities. |
| |
| When a swap page is passed from swapout to zswap, zswap maintains a mapping |
| of the swap entry, a combination of the swap type and swap offset, to the zpool |
| handle that references that compressed swap page. This mapping is achieved |
| with a red-black tree per swap type. The swap offset is the search key for the |
| tree nodes. |
| |
| During a page fault on a PTE that is a swap entry, the swapin code calls the |
| zswap load function to decompress the page into the page allocated by the page |
| fault handler. |
| |
| Once there are no PTEs referencing a swap page stored in zswap (i.e. the count |
| in the swap_map goes to 0) the swap code calls the zswap invalidate function |
| to free the compressed entry. |
| |
| Zswap seeks to be simple in its policies. Sysfs attributes allow for one user |
| controlled policy: |
| |
| * max_pool_percent - The maximum percentage of memory that the compressed |
| pool can occupy. |
| |
| 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.:: |
| |
| echo lzo > /sys/module/zswap/parameters/compressor |
| |
| When the zpool and/or compressor parameter is changed at runtime, any existing |
| compressed pages are not modified; they are left in their own zpool. When a |
| request is made for a page in an old zpool, it is uncompressed using its |
| original compressor. Once all pages are removed from an old zpool, the zpool |
| and its compressor are freed. |
| |
| Some of the pages in zswap are same-value filled pages (i.e. contents of the |
| page have same value or repetitive pattern). These pages include zero-filled |
| pages and they are handled differently. During store operation, a page is |
| checked if it is a same-value filled page before compressing it. If true, the |
| compressed length of the page is set to zero and the pattern or same-filled |
| value is stored. |
| |
| Same-value filled pages identification feature is enabled by default and can be |
| disabled at boot time by setting the ``same_filled_pages_enabled`` attribute |
| to 0, e.g. ``zswap.same_filled_pages_enabled=0``. It can also be enabled and |
| disabled at runtime using the sysfs ``same_filled_pages_enabled`` |
| 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. |
| 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 |
| 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_threshold_percent`` |
| attribute, e. g.:: |
| |
| echo 80 > /sys/module/zswap/parameters/accept_threshold_percent |
| |
| Setting this parameter to 100 will disable the hysteresis. |
| |
| 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. |