83 lines
4.0 KiB
Plaintext
83 lines
4.0 KiB
Plaintext
Overview:
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Zswap is a lightweight compressed cache for swap pages. It takes pages that are
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in the process of being swapped out and attempts to compress them into a
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dynamically allocated RAM-based memory pool. zswap basically trades CPU cycles
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for potentially reduced swap I/O. This trade-off can also result in a
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significant performance improvement if reads from the compressed cache are
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faster than reads from a swap device.
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NOTE: Zswap is a new feature as of v3.11 and interacts heavily with memory
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reclaim. This interaction has not been fully explored on the large set of
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potential configurations and workloads that exist. For this reason, zswap
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is a work in progress and should be considered experimental.
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Some potential benefits:
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* Desktop/laptop users with limited RAM capacities can mitigate the
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performance impact of swapping.
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* Overcommitted guests that share a common I/O resource can
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dramatically reduce their swap I/O pressure, avoiding heavy handed I/O
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throttling by the hypervisor. This allows more work to get done with less
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impact to the guest workload and guests sharing the I/O subsystem
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* Users with SSDs as swap devices can extend the life of the device by
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drastically reducing life-shortening writes.
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Zswap evicts pages from compressed cache on an LRU basis to the backing swap
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device when the compressed pool reaches its size limit. This requirement had
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been identified in prior community discussions.
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Zswap is disabled by default but can be enabled at boot time by setting
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the "enabled" attribute to 1 at boot time. ie: zswap.enabled=1. Zswap
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can also be enabled and disabled at runtime using the sysfs interface.
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An example command to enable zswap at runtime, assuming sysfs is mounted
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at /sys, is:
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echo 1 > /sys/modules/zswap/parameters/enabled
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When zswap is disabled at runtime it will stop storing pages that are
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being swapped out. However, it will _not_ immediately write out or fault
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back into memory all of the pages stored in the compressed pool. The
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pages stored in zswap will remain in the compressed pool until they are
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either invalidated or faulted back into memory. In order to force all
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pages out of the compressed pool, a swapoff on the swap device(s) will
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fault back into memory all swapped out pages, including those in the
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compressed pool.
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Design:
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Zswap receives pages for compression through the Frontswap API and is able to
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evict pages from its own compressed pool on an LRU basis and write them back to
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the backing swap device in the case that the compressed pool is full.
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Zswap makes use of zbud for the managing the compressed memory pool. Each
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allocation in zbud is not directly accessible by address. Rather, a handle is
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returned by the allocation routine and that handle must be mapped before being
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accessed. The compressed memory pool grows on demand and shrinks as compressed
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pages are freed. The pool is not preallocated.
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When a swap page is passed from frontswap to zswap, zswap maintains a mapping
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of the swap entry, a combination of the swap type and swap offset, to the zbud
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handle that references that compressed swap page. This mapping is achieved
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with a red-black tree per swap type. The swap offset is the search key for the
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tree nodes.
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During a page fault on a PTE that is a swap entry, frontswap calls the zswap
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load function to decompress the page into the page allocated by the page fault
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handler.
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Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
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in the swap_map goes to 0) the swap code calls the zswap invalidate function,
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via frontswap, to free the compressed entry.
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Zswap seeks to be simple in its policies. Sysfs attributes allow for one user
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controlled policy:
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* max_pool_percent - The maximum percentage of memory that the compressed
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pool can occupy.
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Zswap allows the compressor to be selected at kernel boot time by setting the
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“compressor” attribute. The default compressor is lzo. e.g.
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zswap.compressor=deflate
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A debugfs interface is provided for various statistic about pool size, number
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of pages stored, and various counters for the reasons pages are rejected.
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