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    Subject[PATCHv6 8/8] zswap: add documentation
    Date
    This patch adds the documentation file for the zswap functionality

    Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
    ---
    Documentation/vm/zswap.txt | 82 ++++++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 82 insertions(+)
    create mode 100644 Documentation/vm/zswap.txt

    diff --git a/Documentation/vm/zswap.txt b/Documentation/vm/zswap.txt
    new file mode 100644
    index 0000000..f29b82f
    --- /dev/null
    +++ b/Documentation/vm/zswap.txt
    @@ -0,0 +1,82 @@
    +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.
    +If this process is successful, the writeback to the swap device is
    +deferred and, in many cases, avoided completely.  This results in
    +a significant I/O reduction and performance gains for systems that
    +are swapping.
    +
    +Zswap provides compressed swap caching that basically trades CPU cycles
    +for reduced swap I/O.  This trade-off can result in a significant
    +performance improvement as reads to/writes from to the compressed
    +cache almost always faster that reading from a swap device
    +which incurs the latency of an asynchronous block I/O read.
    +
    +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 compress pool reaches it size limit or the pool is
    +unable to obtain additional pages from the buddy allocator.  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 or unable to secure additional pages from
    +the buddy allocator.
    +
    +Zswap makes use of zsmalloc for the managing the compressed memory
    +pool. This is because zsmalloc is specifically designed to minimize
    +fragmentation on large (> PAGE_SIZE/2) allocation sizes. Each
    +allocation in zsmalloc is not directly accessible by address.
    +Rather, a handle is return 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 zsmalloc 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
    +two user controlled policies:
    +* max_compression_ratio - Maximum compression ratio, as as percentage,
    + for an acceptable compressed page. Any page that does not compress
    + by at least this ratio will be rejected.
    +* 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.
    --
    1.8.1.1
    --
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