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Cleancache is a new optional feature provided by the VFS layer that
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potentially dramatically increases page cache effectiveness for
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many workloads in many environments at a negligible cost.
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Cleancache can be thought of as a page-granularity victim cache for clean
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pages that the kernel's pageframe replacement algorithm (PFRA) would like
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to keep around, but can't since there isn't enough memory. So when the
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PFRA "evicts" a page, it first attempts to use cleancache code to
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put the data contained in that page into "transcendent memory", memory
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that is not directly accessible or addressable by the kernel and is
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of unknown and possibly time-varying size.
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Later, when a cleancache-enabled filesystem wishes to access a page
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in a file on disk, it first checks cleancache to see if it already
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contains it; if it does, the page of data is copied into the kernel
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and a disk access is avoided.
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Transcendent memory "drivers" for cleancache are currently implemented
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in Xen (using hypervisor memory) and zcache (using in-kernel compressed
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memory) and other implementations are in development.
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FAQs are included below.
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IMPLEMENTATION OVERVIEW
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A cleancache "backend" that provides transcendent memory registers itself
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to the kernel's cleancache "frontend" by calling cleancache_register_ops,
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passing a pointer to a cleancache_ops structure with funcs set appropriately.
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Note that cleancache_register_ops returns the previous settings so that
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chaining can be performed if desired. The functions provided must conform to
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certain semantics as follows:
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Most important, cleancache is "ephemeral". Pages which are copied into
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cleancache have an indefinite lifetime which is completely unknowable
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by the kernel and so may or may not still be in cleancache at any later time.
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Thus, as its name implies, cleancache is not suitable for dirty pages.
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Cleancache has complete discretion over what pages to preserve and what
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pages to discard and when.
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Mounting a cleancache-enabled filesystem should call "init_fs" to obtain a
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pool id which, if positive, must be saved in the filesystem's superblock;
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a negative return value indicates failure. A "put_page" will copy a
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(presumably about-to-be-evicted) page into cleancache and associate it with
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the pool id, a file key, and a page index into the file. (The combination
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of a pool id, a file key, and an index is sometimes called a "handle".)
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A "get_page" will copy the page, if found, from cleancache into kernel memory.
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A "flush_page" will ensure the page no longer is present in cleancache;
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a "flush_inode" will flush all pages associated with the specified file;
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and, when a filesystem is unmounted, a "flush_fs" will flush all pages in
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all files specified by the given pool id and also surrender the pool id.
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An "init_shared_fs", like init_fs, obtains a pool id but tells cleancache
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to treat the pool as shared using a 128-bit UUID as a key. On systems
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that may run multiple kernels (such as hard partitioned or virtualized
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systems) that may share a clustered filesystem, and where cleancache
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may be shared among those kernels, calls to init_shared_fs that specify the
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same UUID will receive the same pool id, thus allowing the pages to
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be shared. Note that any security requirements must be imposed outside
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of the kernel (e.g. by "tools" that control cleancache). Or a
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cleancache implementation can simply disable shared_init by always
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returning a negative value.
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If a get_page is successful on a non-shared pool, the page is flushed (thus
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making cleancache an "exclusive" cache). On a shared pool, the page
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is NOT flushed on a successful get_page so that it remains accessible to
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other sharers. The kernel is responsible for ensuring coherency between
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cleancache (shared or not), the page cache, and the filesystem, using
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cleancache flush operations as required.
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Note that cleancache must enforce put-put-get coherency and get-get
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coherency. For the former, if two puts are made to the same handle but
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with different data, say AAA by the first put and BBB by the second, a
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subsequent get can never return the stale data (AAA). For get-get coherency,
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if a get for a given handle fails, subsequent gets for that handle will
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never succeed unless preceded by a successful put with that handle.
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Last, cleancache provides no SMP serialization guarantees; if two
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different Linux threads are simultaneously putting and flushing a page
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with the same handle, the results are indeterminate. Callers must
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lock the page to ensure serial behavior.
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CLEANCACHE PERFORMANCE METRICS
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Cleancache monitoring is done by sysfs files in the
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/sys/kernel/mm/cleancache directory. The effectiveness of cleancache
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can be measured (across all filesystems) with:
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succ_gets - number of gets that were successful
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failed_gets - number of gets that failed
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puts - number of puts attempted (all "succeed")
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flushes - number of flushes attempted
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A backend implementatation may provide additional metrics.
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1) Where's the value? (Andrew Morton)
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Cleancache provides a significant performance benefit to many workloads
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in many environments with negligible overhead by improving the
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effectiveness of the pagecache. Clean pagecache pages are
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saved in transcendent memory (RAM that is otherwise not directly
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addressable to the kernel); fetching those pages later avoids "refaults"
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Cleancache (and its sister code "frontswap") provide interfaces for
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this transcendent memory (aka "tmem"), which conceptually lies between
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fast kernel-directly-addressable RAM and slower DMA/asynchronous devices.
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Disallowing direct kernel or userland reads/writes to tmem
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is ideal when data is transformed to a different form and size (such
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as with compression) or secretly moved (as might be useful for write-
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balancing for some RAM-like devices). Evicted page-cache pages (and
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swap pages) are a great use for this kind of slower-than-RAM-but-much-
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faster-than-disk transcendent memory, and the cleancache (and frontswap)
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"page-object-oriented" specification provides a nice way to read and
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write -- and indirectly "name" -- the pages.
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In the virtual case, the whole point of virtualization is to statistically
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multiplex physical resources across the varying demands of multiple
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virtual machines. This is really hard to do with RAM and efforts to
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do it well with no kernel change have essentially failed (except in some
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well-publicized special-case workloads). Cleancache -- and frontswap --
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with a fairly small impact on the kernel, provide a huge amount
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of flexibility for more dynamic, flexible RAM multiplexing.
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Specifically, the Xen Transcendent Memory backend allows otherwise
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"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple
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virtual machines, but the pages can be compressed and deduplicated to
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optimize RAM utilization. And when guest OS's are induced to surrender
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underutilized RAM (e.g. with "self-ballooning"), page cache pages
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are the first to go, and cleancache allows those pages to be
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saved and reclaimed if overall host system memory conditions allow.
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And the identical interface used for cleancache can be used in
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physical systems as well. The zcache driver acts as a memory-hungry
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device that stores pages of data in a compressed state. And
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the proposed "RAMster" driver shares RAM across multiple physical
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2) Why does cleancache have its sticky fingers so deep inside the
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filesystems and VFS? (Andrew Morton and Christoph Hellwig)
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The core hooks for cleancache in VFS are in most cases a single line
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and the minimum set are placed precisely where needed to maintain
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coherency (via cleancache_flush operations) between cleancache,
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the page cache, and disk. All hooks compile into nothingness if
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cleancache is config'ed off and turn into a function-pointer-
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compare-to-NULL if config'ed on but no backend claims the ops
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functions, or to a compare-struct-element-to-negative if a
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backend claims the ops functions but a filesystem doesn't enable
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Some filesystems are built entirely on top of VFS and the hooks
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in VFS are sufficient, so don't require an "init_fs" hook; the
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initial implementation of cleancache didn't provide this hook.
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But for some filesystems (such as btrfs), the VFS hooks are
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incomplete and one or more hooks in fs-specific code are required.
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And for some other filesystems, such as tmpfs, cleancache may
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be counterproductive. So it seemed prudent to require a filesystem
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to "opt in" to use cleancache, which requires adding a hook in
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each filesystem. Not all filesystems are supported by cleancache
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only because they haven't been tested. The existing set should
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be sufficient to validate the concept, the opt-in approach means
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that untested filesystems are not affected, and the hooks in the
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existing filesystems should make it very easy to add more
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filesystems in the future.
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The total impact of the hooks to existing fs and mm files is only
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about 40 lines added (not counting comments and blank lines).
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3) Why not make cleancache asynchronous and batched so it can
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more easily interface with real devices with DMA instead
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of copying each individual page? (Minchan Kim)
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The one-page-at-a-time copy semantics simplifies the implementation
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on both the frontend and backend and also allows the backend to
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do fancy things on-the-fly like page compression and
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page deduplication. And since the data is "gone" (copied into/out
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of the pageframe) before the cleancache get/put call returns,
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a great deal of race conditions and potential coherency issues
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are avoided. While the interface seems odd for a "real device"
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or for real kernel-addressable RAM, it makes perfect sense for
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4) Why is non-shared cleancache "exclusive"? And where is the
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page "flushed" after a "get"? (Minchan Kim)
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The main reason is to free up space in transcendent memory and
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to avoid unnecessary cleancache_flush calls. If you want inclusive,
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the page can be "put" immediately following the "get". If
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put-after-get for inclusive becomes common, the interface could
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be easily extended to add a "get_no_flush" call.
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The flush is done by the cleancache backend implementation.
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5) What's the performance impact?
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Performance analysis has been presented at OLS'09 and LCA'10.
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Briefly, performance gains can be significant on most workloads,
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especially when memory pressure is high (e.g. when RAM is
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overcommitted in a virtual workload); and because the hooks are
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invoked primarily in place of or in addition to a disk read/write,
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overhead is negligible even in worst case workloads. Basically
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cleancache replaces I/O with memory-copy-CPU-overhead; on older
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single-core systems with slow memory-copy speeds, cleancache
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has little value, but in newer multicore machines, especially
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consolidated/virtualized machines, it has great value.
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6) How do I add cleancache support for filesystem X? (Boaz Harrash)
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Filesystems that are well-behaved and conform to certain
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restrictions can utilize cleancache simply by making a call to
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cleancache_init_fs at mount time. Unusual, misbehaving, or
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poorly layered filesystems must either add additional hooks
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and/or undergo extensive additional testing... or should just
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not enable the optional cleancache.
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Some points for a filesystem to consider:
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- The FS should be block-device-based (e.g. a ram-based FS such
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as tmpfs should not enable cleancache)
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- To ensure coherency/correctness, the FS must ensure that all
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file removal or truncation operations either go through VFS or
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add hooks to do the equivalent cleancache "flush" operations
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- To ensure coherency/correctness, either inode numbers must
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be unique across the lifetime of the on-disk file OR the
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FS must provide an "encode_fh" function.
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- The FS must call the VFS superblock alloc and deactivate routines
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or add hooks to do the equivalent cleancache calls done there.
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- To maximize performance, all pages fetched from the FS should
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go through the do_mpag_readpage routine or the FS should add
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hooks to do the equivalent (cf. btrfs)
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- Currently, the FS blocksize must be the same as PAGESIZE. This
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is not an architectural restriction, but no backends currently
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support anything different.
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- A clustered FS should invoke the "shared_init_fs" cleancache
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hook to get best performance for some backends.
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7) Why not use the KVA of the inode as the key? (Christoph Hellwig)
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If cleancache would use the inode virtual address instead of
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inode/filehandle, the pool id could be eliminated. But, this
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won't work because cleancache retains pagecache data pages
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persistently even when the inode has been pruned from the
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inode unused list, and only flushes the data page if the file
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gets removed/truncated. So if cleancache used the inode kva,
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there would be potential coherency issues if/when the inode
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kva is reused for a different file. Alternately, if cleancache
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flushed the pages when the inode kva was freed, much of the value
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of cleancache would be lost because the cache of pages in cleanache
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is potentially much larger than the kernel pagecache and is most
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useful if the pages survive inode cache removal.
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8) Why is a global variable required?
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The cleancache_enabled flag is checked in all of the frequently-used
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cleancache hooks. The alternative is a function call to check a static
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variable. Since cleancache is enabled dynamically at runtime, systems
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that don't enable cleancache would suffer thousands (possibly
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tens-of-thousands) of unnecessary function calls per second. So the
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global variable allows cleancache to be enabled by default at compile
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time, but have insignificant performance impact when cleancache remains
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9) Does cleanache work with KVM?
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The memory model of KVM is sufficiently different that a cleancache
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backend may have less value for KVM. This remains to be tested,
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especially in an overcommitted system.
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10) Does cleancache work in userspace? It sounds useful for
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memory hungry caches like web browsers. (Jamie Lokier)
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No plans yet, though we agree it sounds useful, at least for
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apps that bypass the page cache (e.g. O_DIRECT).
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Last updated: Dan Magenheimer, April 13 2011
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removed
Documentation/vm/cleancache.txt
