device when the compressed pool reaches its size limit. This requirement had
been identified in prior community discussions.
-To enabled zswap, the "enabled" attribute must be set to 1 at boot time. e.g.
-zswap.enabled=1
+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.
+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:
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 zbud for the managing the compressed memory pool. Each
-allocation in zbud is not directly accessible by address. Rather, a handle is
+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.
+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.
+
+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. However,
+zsmalloc does not implement compressed page eviction, so once zswap fills it
+cannot evict the oldest page, it can only reject new pages.
When a swap page is passed from frontswap to zswap, zswap maintains a mapping
-of the swap entry, a combination of the swap type and swap offset, to the zbud
+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.
* max_pool_percent - The maximum percentage of memory that the compressed
pool can occupy.
-Zswap allows the compressor to be selected at kernel boot time by setting the
-“compressor” attribute. The default compressor is lzo. e.g.
-zswap.compressor=deflate
+The default compressor is lzo, but it can be selected 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.
A debugfs interface is provided for various statistic about pool size, number
of pages stored, and various counters for the reasons pages are rejected.
Code Review / kvmfornfv.git / blobdiff