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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

3	Cleancache is a new optional feature provided by the VFS layer that
4	potentially dramatically increases page cache effectiveness for
5	many workloads in many environments at a negligible cost.
7	Cleancache can be thought of as a page-granularity victim cache for clean
8	pages that the kernel's pageframe replacement algorithm (PFRA) would like
9	to keep around, but can't since there isn't enough memory.  So when the
10	PFRA "evicts" a page, it first attempts to use cleancache code to
11	put the data contained in that page into "transcendent memory", memory
12	that is not directly accessible or addressable by the kernel and is
13	of unknown and possibly time-varying size.
15	Later, when a cleancache-enabled filesystem wishes to access a page
16	in a file on disk, it first checks cleancache to see if it already
17	contains it; if it does, the page of data is copied into the kernel
18	and a disk access is avoided.
20	Transcendent memory "drivers" for cleancache are currently implemented
21	in Xen (using hypervisor memory) and zcache (using in-kernel compressed
22	memory) and other implementations are in development.
24	FAQs are included below.
28	A cleancache "backend" that provides transcendent memory registers itself
29	to the kernel's cleancache "frontend" by calling cleancache_register_ops,
30	passing a pointer to a cleancache_ops structure with funcs set appropriately.
31	The functions provided must conform to certain semantics as follows:
33	Most important, cleancache is "ephemeral".  Pages which are copied into
34	cleancache have an indefinite lifetime which is completely unknowable
35	by the kernel and so may or may not still be in cleancache at any later time.
36	Thus, as its name implies, cleancache is not suitable for dirty pages.
37	Cleancache has complete discretion over what pages to preserve and what
38	pages to discard and when.
40	Mounting a cleancache-enabled filesystem should call "init_fs" to obtain a
41	pool id which, if positive, must be saved in the filesystem's superblock;
42	a negative return value indicates failure.  A "put_page" will copy a
43	(presumably about-to-be-evicted) page into cleancache and associate it with
44	the pool id, a file key, and a page index into the file.  (The combination
45	of a pool id, a file key, and an index is sometimes called a "handle".)
46	A "get_page" will copy the page, if found, from cleancache into kernel memory.
47	An "invalidate_page" will ensure the page no longer is present in cleancache;
48	an "invalidate_inode" will invalidate all pages associated with the specified
49	file; and, when a filesystem is unmounted, an "invalidate_fs" will invalidate
50	all pages in all files specified by the given pool id and also surrender
51	the pool id.
53	An "init_shared_fs", like init_fs, obtains a pool id but tells cleancache
54	to treat the pool as shared using a 128-bit UUID as a key.  On systems
55	that may run multiple kernels (such as hard partitioned or virtualized
56	systems) that may share a clustered filesystem, and where cleancache
57	may be shared among those kernels, calls to init_shared_fs that specify the
58	same UUID will receive the same pool id, thus allowing the pages to
59	be shared.  Note that any security requirements must be imposed outside
60	of the kernel (e.g. by "tools" that control cleancache).  Or a
61	cleancache implementation can simply disable shared_init by always
62	returning a negative value.
64	If a get_page is successful on a non-shared pool, the page is invalidated
65	(thus making cleancache an "exclusive" cache).  On a shared pool, the page
66	is NOT invalidated on a successful get_page so that it remains accessible to
67	other sharers.  The kernel is responsible for ensuring coherency between
68	cleancache (shared or not), the page cache, and the filesystem, using
69	cleancache invalidate operations as required.
71	Note that cleancache must enforce put-put-get coherency and get-get
72	coherency.  For the former, if two puts are made to the same handle but
73	with different data, say AAA by the first put and BBB by the second, a
74	subsequent get can never return the stale data (AAA).  For get-get coherency,
75	if a get for a given handle fails, subsequent gets for that handle will
76	never succeed unless preceded by a successful put with that handle.
78	Last, cleancache provides no SMP serialization guarantees; if two
79	different Linux threads are simultaneously putting and invalidating a page
80	with the same handle, the results are indeterminate.  Callers must
81	lock the page to ensure serial behavior.
85	If properly configured, monitoring of cleancache is done via debugfs in
86	the /sys/kernel/debug/cleancache directory.  The effectiveness of cleancache
87	can be measured (across all filesystems) with:
89	succ_gets	- number of gets that were successful
90	failed_gets	- number of gets that failed
91	puts		- number of puts attempted (all "succeed")
92	invalidates	- number of invalidates attempted
94	A backend implementation may provide additional metrics.
96	FAQ
98	1) Where's the value? (Andrew Morton)
100	Cleancache provides a significant performance benefit to many workloads
101	in many environments with negligible overhead by improving the
102	effectiveness of the pagecache.  Clean pagecache pages are
103	saved in transcendent memory (RAM that is otherwise not directly
104	addressable to the kernel); fetching those pages later avoids "refaults"
105	and thus disk reads.
107	Cleancache (and its sister code "frontswap") provide interfaces for
108	this transcendent memory (aka "tmem"), which conceptually lies between
109	fast kernel-directly-addressable RAM and slower DMA/asynchronous devices.
110	Disallowing direct kernel or userland reads/writes to tmem
111	is ideal when data is transformed to a different form and size (such
112	as with compression) or secretly moved (as might be useful for write-
113	balancing for some RAM-like devices).  Evicted page-cache pages (and
114	swap pages) are a great use for this kind of slower-than-RAM-but-much-
115	faster-than-disk transcendent memory, and the cleancache (and frontswap)
116	"page-object-oriented" specification provides a nice way to read and
117	write -- and indirectly "name" -- the pages.
119	In the virtual case, the whole point of virtualization is to statistically
120	multiplex physical resources across the varying demands of multiple
121	virtual machines.  This is really hard to do with RAM and efforts to
122	do it well with no kernel change have essentially failed (except in some
123	well-publicized special-case workloads).  Cleancache -- and frontswap --
124	with a fairly small impact on the kernel, provide a huge amount
125	of flexibility for more dynamic, flexible RAM multiplexing.
126	Specifically, the Xen Transcendent Memory backend allows otherwise
127	"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple
128	virtual machines, but the pages can be compressed and deduplicated to
129	optimize RAM utilization.  And when guest OS's are induced to surrender
130	underutilized RAM (e.g. with "self-ballooning"), page cache pages
131	are the first to go, and cleancache allows those pages to be
132	saved and reclaimed if overall host system memory conditions allow.
134	And the identical interface used for cleancache can be used in
135	physical systems as well.  The zcache driver acts as a memory-hungry
136	device that stores pages of data in a compressed state.  And
137	the proposed "RAMster" driver shares RAM across multiple physical
138	systems.
140	2) Why does cleancache have its sticky fingers so deep inside the
141	   filesystems and VFS? (Andrew Morton and Christoph Hellwig)
143	The core hooks for cleancache in VFS are in most cases a single line
144	and the minimum set are placed precisely where needed to maintain
145	coherency (via cleancache_invalidate operations) between cleancache,
146	the page cache, and disk.  All hooks compile into nothingness if
147	cleancache is config'ed off and turn into a function-pointer-
148	compare-to-NULL if config'ed on but no backend claims the ops
149	functions, or to a compare-struct-element-to-negative if a
150	backend claims the ops functions but a filesystem doesn't enable
151	cleancache.
153	Some filesystems are built entirely on top of VFS and the hooks
154	in VFS are sufficient, so don't require an "init_fs" hook; the
155	initial implementation of cleancache didn't provide this hook.
156	But for some filesystems (such as btrfs), the VFS hooks are
157	incomplete and one or more hooks in fs-specific code are required.
158	And for some other filesystems, such as tmpfs, cleancache may
159	be counterproductive.  So it seemed prudent to require a filesystem
160	to "opt in" to use cleancache, which requires adding a hook in
161	each filesystem.  Not all filesystems are supported by cleancache
162	only because they haven't been tested.  The existing set should
163	be sufficient to validate the concept, the opt-in approach means
164	that untested filesystems are not affected, and the hooks in the
165	existing filesystems should make it very easy to add more
166	filesystems in the future.
168	The total impact of the hooks to existing fs and mm files is only
169	about 40 lines added (not counting comments and blank lines).
171	3) Why not make cleancache asynchronous and batched so it can
172	   more easily interface with real devices with DMA instead
173	   of copying each individual page? (Minchan Kim)
175	The one-page-at-a-time copy semantics simplifies the implementation
176	on both the frontend and backend and also allows the backend to
177	do fancy things on-the-fly like page compression and
178	page deduplication.  And since the data is "gone" (copied into/out
179	of the pageframe) before the cleancache get/put call returns,
180	a great deal of race conditions and potential coherency issues
181	are avoided.  While the interface seems odd for a "real device"
182	or for real kernel-addressable RAM, it makes perfect sense for
183	transcendent memory.
185	4) Why is non-shared cleancache "exclusive"?  And where is the
186	   page "invalidated" after a "get"? (Minchan Kim)
188	The main reason is to free up space in transcendent memory and
189	to avoid unnecessary cleancache_invalidate calls.  If you want inclusive,
190	the page can be "put" immediately following the "get".  If
191	put-after-get for inclusive becomes common, the interface could
192	be easily extended to add a "get_no_invalidate" call.
194	The invalidate is done by the cleancache backend implementation.
196	5) What's the performance impact?
198	Performance analysis has been presented at OLS'09 and LCA'10.
199	Briefly, performance gains can be significant on most workloads,
200	especially when memory pressure is high (e.g. when RAM is
201	overcommitted in a virtual workload); and because the hooks are
202	invoked primarily in place of or in addition to a disk read/write,
203	overhead is negligible even in worst case workloads.  Basically
204	cleancache replaces I/O with memory-copy-CPU-overhead; on older
205	single-core systems with slow memory-copy speeds, cleancache
206	has little value, but in newer multicore machines, especially
207	consolidated/virtualized machines, it has great value.
209	6) How do I add cleancache support for filesystem X? (Boaz Harrash)
211	Filesystems that are well-behaved and conform to certain
212	restrictions can utilize cleancache simply by making a call to
213	cleancache_init_fs at mount time.  Unusual, misbehaving, or
214	poorly layered filesystems must either add additional hooks
215	and/or undergo extensive additional testing... or should just
216	not enable the optional cleancache.
218	Some points for a filesystem to consider:
220	- The FS should be block-device-based (e.g. a ram-based FS such
221	  as tmpfs should not enable cleancache)
222	- To ensure coherency/correctness, the FS must ensure that all
223	  file removal or truncation operations either go through VFS or
224	  add hooks to do the equivalent cleancache "invalidate" operations
225	- To ensure coherency/correctness, either inode numbers must
226	  be unique across the lifetime of the on-disk file OR the
227	  FS must provide an "encode_fh" function.
228	- The FS must call the VFS superblock alloc and deactivate routines
229	  or add hooks to do the equivalent cleancache calls done there.
230	- To maximize performance, all pages fetched from the FS should
231	  go through the do_mpag_readpage routine or the FS should add
232	  hooks to do the equivalent (cf. btrfs)
233	- Currently, the FS blocksize must be the same as PAGESIZE.  This
234	  is not an architectural restriction, but no backends currently
235	  support anything different.
236	- A clustered FS should invoke the "shared_init_fs" cleancache
237	  hook to get best performance for some backends.
239	7) Why not use the KVA of the inode as the key? (Christoph Hellwig)
241	If cleancache would use the inode virtual address instead of
242	inode/filehandle, the pool id could be eliminated.  But, this
243	won't work because cleancache retains pagecache data pages
244	persistently even when the inode has been pruned from the
245	inode unused list, and only invalidates the data page if the file
246	gets removed/truncated.  So if cleancache used the inode kva,
247	there would be potential coherency issues if/when the inode
248	kva is reused for a different file.  Alternately, if cleancache
249	invalidated the pages when the inode kva was freed, much of the value
250	of cleancache would be lost because the cache of pages in cleanache
251	is potentially much larger than the kernel pagecache and is most
252	useful if the pages survive inode cache removal.
254	8) Why is a global variable required?
256	The cleancache_enabled flag is checked in all of the frequently-used
257	cleancache hooks.  The alternative is a function call to check a static
258	variable. Since cleancache is enabled dynamically at runtime, systems
259	that don't enable cleancache would suffer thousands (possibly
260	tens-of-thousands) of unnecessary function calls per second.  So the
261	global variable allows cleancache to be enabled by default at compile
262	time, but have insignificant performance impact when cleancache remains
263	disabled at runtime.
265	9) Does cleanache work with KVM?
267	The memory model of KVM is sufficiently different that a cleancache
268	backend may have less value for KVM.  This remains to be tested,
269	especially in an overcommitted system.
271	10) Does cleancache work in userspace?  It sounds useful for
272	   memory hungry caches like web browsers.  (Jamie Lokier)
274	No plans yet, though we agree it sounds useful, at least for
275	apps that bypass the page cache (e.g. O_DIRECT).
277	Last updated: Dan Magenheimer, April 13 2011
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