[V3,1/8] Cleancache: Documentation
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Message ID 20100621231839.GA19454@ca-server1.us.oracle.com
State New, archived
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Dan Magenheimer June 21, 2010, 11:18 p.m. UTC
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--- linux-2.6.35-rc2/Documentation/ABI/testing/sysfs-kernel-mm-cleancache	1969-12-31 17:00:00.000000000 -0700
+++ linux-2.6.35-rc2-cleancache/Documentation/ABI/testing/sysfs-kernel-mm-cleancache	2010-06-11 09:10:25.000000000 -0600
@@ -0,0 +1,11 @@ 
+What:		/sys/kernel/mm/cleancache/
+Date:		June 2010
+Contact:	Dan Magenheimer <dan.magenheimer@oracle.com>
+Description:
+		/sys/kernel/mm/cleancache/ contains a number of files which
+		record a count of various cleancache operations
+		(sum across all filesystems):
+			succ_gets
+			failed_gets
+			puts
+			flushes
--- linux-2.6.35-rc2/Documentation/vm/cleancache.txt	1969-12-31 17:00:00.000000000 -0700
+++ linux-2.6.35-rc2-cleancache/Documentation/vm/cleancache.txt	2010-06-21 16:51:54.000000000 -0600
@@ -0,0 +1,194 @@ 
+MOTIVATION
+
+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 put it into a synchronous
+concurrency-safe page-oriented "pseudo-RAM" device (such as Xen's Transcendent
+Memory, aka "tmem", or in-kernel compressed memory, aka "zmem", or other
+RAM-like devices) which is not directly accessible or addressable by the
+kernel and is of unknown and possibly time-varying size.  And 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 is copied into the kernel and a disk access is avoided.
+
+A FAQ is included below:
+
+IMPLEMENTATION OVERVIEW
+
+A cleancache "backend" that interfaces to this pseudo-RAM links itself
+to the kernel's cleancache "frontend" by setting the cleancache_ops funcs
+appropriately and the functions it provides 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, the file inode, and a page index into the file.  (The combination
+of a pool id, an inode, and an index is sometimes called a "handle".)
+A "get_page" will copy the page, if found, from cleancache into kernel memory.
+A "flush_page" will ensure the page no longer is present in cleancache;
+a "flush_inode" will flush all pages associated with the specified inode;
+and, when a filesystem is unmounted, a "flush_fs" will flush all pages in
+all inodes specified by the given pool id and also surrender the pool id.
+
+A "init_shared_fs", like init, 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 flushed (thus
+making cleancache an "exclusive" cache).  On a shared pool, the page
+is NOT flushed 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 flush 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 flushing a page
+with the same handle, the results are indeterminate.
+
+CLEANCACHE PERFORMANCE METRICS
+
+Cleancache monitoring is done by sysfs files in the
+/sys/kernel/mm/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")
+flushes		- number of flushes attempted
+
+A backend implementatation may provide additional metrics.
+
+FAQ
+
+1) Where's the value? (Andrew Morton)
+
+Cleancache (and its sister code "frontswap") provide interfaces for
+a new pseudo-RAM memory type that conceptually lies between fast
+kernel-directly-addressable RAM and slower DMA/asynchronous devices.
+Disallowing direct kernel or userland reads/writes to this pseudo-RAM
+is ideal when data is transformed to a different form and size (such
+as wiht 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 pseudo-RAM 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.
+
+2) Why does cleancache have its sticky fingers so deep inside the
+   filesystems and VFS? (Andrew Morton and Christophe 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_flush operatings) 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 a "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.
+
+3) 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-addressible RAM, it makes perfect sense for
+pseudo-RAM.
+
+4) Why is non-shared cleancache "exclusive"?  And where is the
+   page "flushed" after a "get"? (Minchan Kim)
+
+The main reason is to free up memory in pseudo-RAM and to avoid
+unnecessary cleancache_flush 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_flush" call.
+
+The flush is done by the cleancache backend implementation.
+
+5) 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.
+
+6) Does cleanache work with KVM?
+
+The memory model of KVM is sufficiently different that a cleancache
+backend may have little value for KVM.  This remains to be tested,
+especially in an overcommitted system.
+
+7) 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, June 21 2010