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diff --git a/Documentation/vm/cleancache.rst b/Documentation/vm/cleancache.rst new file mode 100644 index 000000000000..68cba9131c31 --- /dev/null +++ b/Documentation/vm/cleancache.rst @@ -0,0 +1,296 @@ +.. _cleancache: + +========== +Cleancache +========== + +Motivation +========== + +Cleancache is a new optional feature provided by the VFS layer that +potentially dramatically increases page cache effectiveness for +many workloads in many environments at a negligible cost. + +Cleancache can be thought of as a page-granularity victim cache for clean +pages that the kernel's pageframe replacement algorithm (PFRA) would like +to keep around, but can't since there isn't enough memory. So when the +PFRA "evicts" a page, it first attempts to use cleancache code to +put the data contained in that page into "transcendent memory", memory +that is not directly accessible or addressable by the kernel and is +of unknown and possibly time-varying size. + +Later, when a cleancache-enabled filesystem wishes to access a page +in a file on disk, it first checks cleancache to see if it already +contains it; if it does, the page of data is copied into the kernel +and a disk access is avoided. + +Transcendent memory "drivers" for cleancache are currently implemented +in Xen (using hypervisor memory) and zcache (using in-kernel compressed +memory) and other implementations are in development. + +:ref:`FAQs <faq>` are included below. + +Implementation Overview +======================= + +A cleancache "backend" that provides transcendent memory registers itself +to the kernel's cleancache "frontend" by calling cleancache_register_ops, +passing a pointer to a cleancache_ops structure with funcs set appropriately. +The functions provided must conform to certain semantics as follows: + +Most important, cleancache is "ephemeral". Pages which are copied into +cleancache have an indefinite lifetime which is completely unknowable +by the kernel and so may or may not still be in cleancache at any later time. +Thus, as its name implies, cleancache is not suitable for dirty pages. +Cleancache has complete discretion over what pages to preserve and what +pages to discard and when. + +Mounting a cleancache-enabled filesystem should call "init_fs" to obtain a +pool id which, if positive, must be saved in the filesystem's superblock; +a negative return value indicates failure. A "put_page" will copy a +(presumably about-to-be-evicted) page into cleancache and associate it with +the pool id, a file key, and a page index into the file. (The combination +of a pool id, a file key, and an index is sometimes called a "handle".) +A "get_page" will copy the page, if found, from cleancache into kernel memory. +An "invalidate_page" will ensure the page no longer is present in cleancache; +an "invalidate_inode" will invalidate all pages associated with the specified +file; and, when a filesystem is unmounted, an "invalidate_fs" will invalidate +all pages in all files specified by the given pool id and also surrender +the pool id. + +An "init_shared_fs", like init_fs, obtains a pool id but tells cleancache +to treat the pool as shared using a 128-bit UUID as a key. On systems +that may run multiple kernels (such as hard partitioned or virtualized +systems) that may share a clustered filesystem, and where cleancache +may be shared among those kernels, calls to init_shared_fs that specify the +same UUID will receive the same pool id, thus allowing the pages to +be shared. Note that any security requirements must be imposed outside +of the kernel (e.g. by "tools" that control cleancache). Or a +cleancache implementation can simply disable shared_init by always +returning a negative value. + +If a get_page is successful on a non-shared pool, the page is invalidated +(thus making cleancache an "exclusive" cache). On a shared pool, the page +is NOT invalidated on a successful get_page so that it remains accessible to +other sharers. The kernel is responsible for ensuring coherency between +cleancache (shared or not), the page cache, and the filesystem, using +cleancache invalidate operations as required. + +Note that cleancache must enforce put-put-get coherency and get-get +coherency. For the former, if two puts are made to the same handle but +with different data, say AAA by the first put and BBB by the second, a +subsequent get can never return the stale data (AAA). For get-get coherency, +if a get for a given handle fails, subsequent gets for that handle will +never succeed unless preceded by a successful put with that handle. + +Last, cleancache provides no SMP serialization guarantees; if two +different Linux threads are simultaneously putting and invalidating a page +with the same handle, the results are indeterminate. Callers must +lock the page to ensure serial behavior. + +Cleancache Performance Metrics +============================== + +If properly configured, monitoring of cleancache is done via debugfs in +the `/sys/kernel/debug/cleancache` directory. The effectiveness of cleancache +can be measured (across all filesystems) with: + +``succ_gets`` + number of gets that were successful + +``failed_gets`` + number of gets that failed + +``puts`` + number of puts attempted (all "succeed") + +``invalidates`` + number of invalidates attempted + +A backend implementation may provide additional metrics. + +.. _faq: + +FAQ +=== + +* Where's the value? (Andrew Morton) + +Cleancache provides a significant performance benefit to many workloads +in many environments with negligible overhead by improving the +effectiveness of the pagecache. Clean pagecache pages are +saved in transcendent memory (RAM that is otherwise not directly +addressable to the kernel); fetching those pages later avoids "refaults" +and thus disk reads. + +Cleancache (and its sister code "frontswap") provide interfaces for +this transcendent memory (aka "tmem"), which conceptually lies between +fast kernel-directly-addressable RAM and slower DMA/asynchronous devices. +Disallowing direct kernel or userland reads/writes to tmem +is ideal when data is transformed to a different form and size (such +as with compression) or secretly moved (as might be useful for write- +balancing for some RAM-like devices). Evicted page-cache pages (and +swap pages) are a great use for this kind of slower-than-RAM-but-much- +faster-than-disk transcendent memory, and the cleancache (and frontswap) +"page-object-oriented" specification provides a nice way to read and +write -- and indirectly "name" -- the pages. + +In the virtual case, the whole point of virtualization is to statistically +multiplex physical resources across the varying demands of multiple +virtual machines. This is really hard to do with RAM and efforts to +do it well with no kernel change have essentially failed (except in some +well-publicized special-case workloads). Cleancache -- and frontswap -- +with a fairly small impact on the kernel, provide a huge amount +of flexibility for more dynamic, flexible RAM multiplexing. +Specifically, the Xen Transcendent Memory backend allows otherwise +"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple +virtual machines, but the pages can be compressed and deduplicated to +optimize RAM utilization. And when guest OS's are induced to surrender +underutilized RAM (e.g. with "self-ballooning"), page cache pages +are the first to go, and cleancache allows those pages to be +saved and reclaimed if overall host system memory conditions allow. + +And the identical interface used for cleancache can be used in +physical systems as well. The zcache driver acts as a memory-hungry +device that stores pages of data in a compressed state. And +the proposed "RAMster" driver shares RAM across multiple physical +systems. + +* Why does cleancache have its sticky fingers so deep inside the + filesystems and VFS? (Andrew Morton and Christoph Hellwig) + +The core hooks for cleancache in VFS are in most cases a single line +and the minimum set are placed precisely where needed to maintain +coherency (via cleancache_invalidate operations) between cleancache, +the page cache, and disk. All hooks compile into nothingness if +cleancache is config'ed off and turn into a function-pointer- +compare-to-NULL if config'ed on but no backend claims the ops +functions, or to a compare-struct-element-to-negative if a +backend claims the ops functions but a filesystem doesn't enable +cleancache. + +Some filesystems are built entirely on top of VFS and the hooks +in VFS are sufficient, so don't require an "init_fs" hook; the +initial implementation of cleancache didn't provide this hook. +But for some filesystems (such as btrfs), the VFS hooks are +incomplete and one or more hooks in fs-specific code are required. +And for some other filesystems, such as tmpfs, cleancache may +be counterproductive. So it seemed prudent to require a filesystem +to "opt in" to use cleancache, which requires adding a hook in +each filesystem. Not all filesystems are supported by cleancache +only because they haven't been tested. The existing set should +be sufficient to validate the concept, the opt-in approach means +that untested filesystems are not affected, and the hooks in the +existing filesystems should make it very easy to add more +filesystems in the future. + +The total impact of the hooks to existing fs and mm files is only +about 40 lines added (not counting comments and blank lines). + +* Why not make cleancache asynchronous and batched so it can more + easily interface with real devices with DMA instead of copying each + individual page? (Minchan Kim) + +The one-page-at-a-time copy semantics simplifies the implementation +on both the frontend and backend and also allows the backend to +do fancy things on-the-fly like page compression and +page deduplication. And since the data is "gone" (copied into/out +of the pageframe) before the cleancache get/put call returns, +a great deal of race conditions and potential coherency issues +are avoided. While the interface seems odd for a "real device" +or for real kernel-addressable RAM, it makes perfect sense for +transcendent memory. + +* Why is non-shared cleancache "exclusive"? And where is the + page "invalidated" after a "get"? (Minchan Kim) + +The main reason is to free up space in transcendent memory and +to avoid unnecessary cleancache_invalidate calls. If you want inclusive, +the page can be "put" immediately following the "get". If +put-after-get for inclusive becomes common, the interface could +be easily extended to add a "get_no_invalidate" call. + +The invalidate is done by the cleancache backend implementation. + +* What's the performance impact? + +Performance analysis has been presented at OLS'09 and LCA'10. +Briefly, performance gains can be significant on most workloads, +especially when memory pressure is high (e.g. when RAM is +overcommitted in a virtual workload); and because the hooks are +invoked primarily in place of or in addition to a disk read/write, +overhead is negligible even in worst case workloads. Basically +cleancache replaces I/O with memory-copy-CPU-overhead; on older +single-core systems with slow memory-copy speeds, cleancache +has little value, but in newer multicore machines, especially +consolidated/virtualized machines, it has great value. + +* How do I add cleancache support for filesystem X? (Boaz Harrash) + +Filesystems that are well-behaved and conform to certain +restrictions can utilize cleancache simply by making a call to +cleancache_init_fs at mount time. Unusual, misbehaving, or +poorly layered filesystems must either add additional hooks +and/or undergo extensive additional testing... or should just +not enable the optional cleancache. + +Some points for a filesystem to consider: + + - The FS should be block-device-based (e.g. a ram-based FS such + as tmpfs should not enable cleancache) + - To ensure coherency/correctness, the FS must ensure that all + file removal or truncation operations either go through VFS or + add hooks to do the equivalent cleancache "invalidate" operations + - To ensure coherency/correctness, either inode numbers must + be unique across the lifetime of the on-disk file OR the + FS must provide an "encode_fh" function. + - The FS must call the VFS superblock alloc and deactivate routines + or add hooks to do the equivalent cleancache calls done there. + - To maximize performance, all pages fetched from the FS should + go through the do_mpag_readpage routine or the FS should add + hooks to do the equivalent (cf. btrfs) + - Currently, the FS blocksize must be the same as PAGESIZE. This + is not an architectural restriction, but no backends currently + support anything different. + - A clustered FS should invoke the "shared_init_fs" cleancache + hook to get best performance for some backends. + +* Why not use the KVA of the inode as the key? (Christoph Hellwig) + +If cleancache would use the inode virtual address instead of +inode/filehandle, the pool id could be eliminated. But, this +won't work because cleancache retains pagecache data pages +persistently even when the inode has been pruned from the +inode unused list, and only invalidates the data page if the file +gets removed/truncated. So if cleancache used the inode kva, +there would be potential coherency issues if/when the inode +kva is reused for a different file. Alternately, if cleancache +invalidated the pages when the inode kva was freed, much of the value +of cleancache would be lost because the cache of pages in cleanache +is potentially much larger than the kernel pagecache and is most +useful if the pages survive inode cache removal. + +* Why is a global variable required? + +The cleancache_enabled flag is checked in all of the frequently-used +cleancache hooks. The alternative is a function call to check a static +variable. Since cleancache is enabled dynamically at runtime, systems +that don't enable cleancache would suffer thousands (possibly +tens-of-thousands) of unnecessary function calls per second. So the +global variable allows cleancache to be enabled by default at compile +time, but have insignificant performance impact when cleancache remains +disabled at runtime. + +* Does cleanache work with KVM? + +The memory model of KVM is sufficiently different that a cleancache +backend may have less value for KVM. This remains to be tested, +especially in an overcommitted system. + +* Does cleancache work in userspace? It sounds useful for + memory hungry caches like web browsers. (Jamie Lokier) + +No plans yet, though we agree it sounds useful, at least for +apps that bypass the page cache (e.g. O_DIRECT). + +Last updated: Dan Magenheimer, April 13 2011 |