169 lines
8.0 KiB
ReStructuredText
169 lines
8.0 KiB
ReStructuredText
.. _zswap:
|
|
|
|
=====
|
|
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.
|
|
|
|
.. 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.
|
|
|
|
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 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 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. 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 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.
|