69 строки
3.3 KiB
Plaintext
69 строки
3.3 KiB
Plaintext
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.
|
||
|
||
To enabled zswap, the "enabled" attribute must be set to 1 at boot time. e.g.
|
||
zswap.enabled=1
|
||
|
||
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 zbud for the managing the compressed memory pool. Each
|
||
allocation in zbud 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.
|
||
|
||
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
|
||
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.
|
||
|
||
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
|
||
|
||
A debugfs interface is provided for various statistic about pool size, number
|
||
of pages stored, and various counters for the reasons pages are rejected.
|