This patch adds the documentation file for the zswap functionality Signed-off-by: Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx> --- 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. -- 1.8.2 _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
