mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-26 02:39:48 +00:00
![Srividya Desireddy](/assets/img/avatar_default.png)
Update zswap document with details on same-value filled pages identification feature. The usage of zswap.same_filled_pages_enabled module parameter is explained. Link: http://lkml.kernel.org/r/20171206114852epcms5p6973b02a9f455d5d3c765eafda0fe2631@epcms5p6 Signed-off-by: Srividya Desireddy <srividya.dr@samsung.com> Acked-by: Dan Streetman <ddstreet@ieee.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
122 lines
6.2 KiB
Text
122 lines
6.2 KiB
Text
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.
|
||
|
||
NOTE: Zswap is a new feature as of v3.11 and interacts heavily with memory
|
||
reclaim. This interaction has not been fully explored on the large set of
|
||
potential configurations and workloads that exist. For this reason, zswap
|
||
is a work in progress and should be considered experimental.
|
||
|
||
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.
|
||
|
||
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:
|
||
|
||
Zswap receives pages for compression through the Frontswap API 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
|
||
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 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, frontswap 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,
|
||
via frontswap, 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 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.
|
||
|
||
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. However, the
|
||
existing pages which are marked as same-value filled pages remain stored
|
||
unchanged in zswap until they are either loaded or invalidated.
|
||
|
||
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.
|