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docs/vm: cleancache.txt: convert to ReST format

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Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mike Rapoport
2018-03-21 21:22:19 +02:00
committed by Jonathan Corbet
parent d04f9f5a78
commit 5ef829e056
1 changed files with 62 additions and 43 deletions
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105
Documentation/vm/cleancache.txt
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@@ -1,4 +1,11 @@
MOTIVATION .. _cleancache:
==========
Cleancache
==========
Motivation
==========
Cleancache is a new optional feature provided by the VFS layer that Cleancache is a new optional feature provided by the VFS layer that
potentially dramatically increases page cache effectiveness for potentially dramatically increases page cache effectiveness for
@@ -21,9 +28,10 @@ Transcendent memory "drivers" for cleancache are currently implemented
in Xen (using hypervisor memory) and zcache (using in-kernel compressed in Xen (using hypervisor memory) and zcache (using in-kernel compressed
memory) and other implementations are in development. memory) and other implementations are in development.
FAQs are included below. :ref:`FAQs <faq>` are included below.
IMPLEMENTATION OVERVIEW Implementation Overview
=======================
A cleancache "backend" that provides transcendent memory registers itself A cleancache "backend" that provides transcendent memory registers itself
to the kernel's cleancache "frontend" by calling cleancache_register_ops, to the kernel's cleancache "frontend" by calling cleancache_register_ops,
@@ -80,22 +88,33 @@ different Linux threads are simultaneously putting and invalidating a page
with the same handle, the results are indeterminate. Callers must with the same handle, the results are indeterminate. Callers must
lock the page to ensure serial behavior. lock the page to ensure serial behavior.
CLEANCACHE PERFORMANCE METRICS Cleancache Performance Metrics
==============================
If properly configured, monitoring of cleancache is done via debugfs in If properly configured, monitoring of cleancache is done via debugfs in
the /sys/kernel/debug/cleancache directory. The effectiveness of cleancache the `/sys/kernel/debug/cleancache` directory. The effectiveness of cleancache
can be measured (across all filesystems) with: can be measured (across all filesystems) with:
succ_gets - number of gets that were successful ``succ_gets``
failed_gets - number of gets that failed number of gets that were successful
puts - number of puts attempted (all "succeed")
invalidates - number of invalidates attempted ``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. A backend implementation may provide additional metrics.
FAQ .. _faq:
1) Where's the value? (Andrew Morton) FAQ
===
* Where's the value? (Andrew Morton)
Cleancache provides a significant performance benefit to many workloads Cleancache provides a significant performance benefit to many workloads
in many environments with negligible overhead by improving the in many environments with negligible overhead by improving the
@@ -137,8 +156,8 @@ device that stores pages of data in a compressed state. And
the proposed "RAMster" driver shares RAM across multiple physical the proposed "RAMster" driver shares RAM across multiple physical
systems. systems.
2) Why does cleancache have its sticky fingers so deep inside the * Why does cleancache have its sticky fingers so deep inside the
filesystems and VFS? (Andrew Morton and Christoph Hellwig) filesystems and VFS? (Andrew Morton and Christoph Hellwig)
The core hooks for cleancache in VFS are in most cases a single line 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 and the minimum set are placed precisely where needed to maintain
@@ -168,9 +187,9 @@ filesystems in the future.
The total impact of the hooks to existing fs and mm files is only The total impact of the hooks to existing fs and mm files is only
about 40 lines added (not counting comments and blank lines). about 40 lines added (not counting comments and blank lines).
3) Why not make cleancache asynchronous and batched so it can * Why not make cleancache asynchronous and batched so it can more
more easily interface with real devices with DMA instead easily interface with real devices with DMA instead of copying each
of copying each individual page? (Minchan Kim) individual page? (Minchan Kim)
The one-page-at-a-time copy semantics simplifies the implementation The one-page-at-a-time copy semantics simplifies the implementation
on both the frontend and backend and also allows the backend to on both the frontend and backend and also allows the backend to
@@ -182,8 +201,8 @@ are avoided. While the interface seems odd for a "real device"
or for real kernel-addressable RAM, it makes perfect sense for or for real kernel-addressable RAM, it makes perfect sense for
transcendent memory. transcendent memory.
4) Why is non-shared cleancache "exclusive"? And where is the * Why is non-shared cleancache "exclusive"? And where is the
page "invalidated" after a "get"? (Minchan Kim) page "invalidated" after a "get"? (Minchan Kim)
The main reason is to free up space in transcendent memory and The main reason is to free up space in transcendent memory and
to avoid unnecessary cleancache_invalidate calls. If you want inclusive, to avoid unnecessary cleancache_invalidate calls. If you want inclusive,
@@ -193,7 +212,7 @@ be easily extended to add a "get_no_invalidate" call.
The invalidate is done by the cleancache backend implementation. The invalidate is done by the cleancache backend implementation.
5) What's the performance impact? * What's the performance impact?
Performance analysis has been presented at OLS'09 and LCA'10. Performance analysis has been presented at OLS'09 and LCA'10.
Briefly, performance gains can be significant on most workloads, Briefly, performance gains can be significant on most workloads,
@@ -206,7 +225,7 @@ single-core systems with slow memory-copy speeds, cleancache
has little value, but in newer multicore machines, especially has little value, but in newer multicore machines, especially
consolidated/virtualized machines, it has great value. consolidated/virtualized machines, it has great value.
6) How do I add cleancache support for filesystem X? (Boaz Harrash) * How do I add cleancache support for filesystem X? (Boaz Harrash)
Filesystems that are well-behaved and conform to certain Filesystems that are well-behaved and conform to certain
restrictions can utilize cleancache simply by making a call to restrictions can utilize cleancache simply by making a call to
@@ -217,26 +236,26 @@ not enable the optional cleancache.
Some points for a filesystem to consider: Some points for a filesystem to consider:
- The FS should be block-device-based (e.g. a ram-based FS such - The FS should be block-device-based (e.g. a ram-based FS such
as tmpfs should not enable cleancache) as tmpfs should not enable cleancache)
- To ensure coherency/correctness, the FS must ensure that all - To ensure coherency/correctness, the FS must ensure that all
file removal or truncation operations either go through VFS or file removal or truncation operations either go through VFS or
add hooks to do the equivalent cleancache "invalidate" operations add hooks to do the equivalent cleancache "invalidate" operations
- To ensure coherency/correctness, either inode numbers must - To ensure coherency/correctness, either inode numbers must
be unique across the lifetime of the on-disk file OR the be unique across the lifetime of the on-disk file OR the
FS must provide an "encode_fh" function. FS must provide an "encode_fh" function.
- The FS must call the VFS superblock alloc and deactivate routines - The FS must call the VFS superblock alloc and deactivate routines
or add hooks to do the equivalent cleancache calls done there. or add hooks to do the equivalent cleancache calls done there.
- To maximize performance, all pages fetched from the FS should - To maximize performance, all pages fetched from the FS should
go through the do_mpag_readpage routine or the FS should add go through the do_mpag_readpage routine or the FS should add
hooks to do the equivalent (cf. btrfs) hooks to do the equivalent (cf. btrfs)
- Currently, the FS blocksize must be the same as PAGESIZE. This - Currently, the FS blocksize must be the same as PAGESIZE. This
is not an architectural restriction, but no backends currently is not an architectural restriction, but no backends currently
support anything different. support anything different.
- A clustered FS should invoke the "shared_init_fs" cleancache - A clustered FS should invoke the "shared_init_fs" cleancache
hook to get best performance for some backends. hook to get best performance for some backends.
7) Why not use the KVA of the inode as the key? (Christoph Hellwig) * Why not use the KVA of the inode as the key? (Christoph Hellwig)
If cleancache would use the inode virtual address instead of If cleancache would use the inode virtual address instead of
inode/filehandle, the pool id could be eliminated. But, this inode/filehandle, the pool id could be eliminated. But, this
@@ -251,7 +270,7 @@ of cleancache would be lost because the cache of pages in cleanache
is potentially much larger than the kernel pagecache and is most is potentially much larger than the kernel pagecache and is most
useful if the pages survive inode cache removal. useful if the pages survive inode cache removal.
8) Why is a global variable required? * Why is a global variable required?
The cleancache_enabled flag is checked in all of the frequently-used The cleancache_enabled flag is checked in all of the frequently-used
cleancache hooks. The alternative is a function call to check a static cleancache hooks. The alternative is a function call to check a static
@@ -262,14 +281,14 @@ global variable allows cleancache to be enabled by default at compile
time, but have insignificant performance impact when cleancache remains time, but have insignificant performance impact when cleancache remains
disabled at runtime. disabled at runtime.
9) Does cleanache work with KVM? * Does cleanache work with KVM?
The memory model of KVM is sufficiently different that a cleancache The memory model of KVM is sufficiently different that a cleancache
backend may have less value for KVM. This remains to be tested, backend may have less value for KVM. This remains to be tested,
especially in an overcommitted system. especially in an overcommitted system.
10) Does cleancache work in userspace? It sounds useful for * Does cleancache work in userspace? It sounds useful for
memory hungry caches like web browsers. (Jamie Lokier) memory hungry caches like web browsers. (Jamie Lokier)
No plans yet, though we agree it sounds useful, at least for No plans yet, though we agree it sounds useful, at least for
apps that bypass the page cache (e.g. O_DIRECT). apps that bypass the page cache (e.g. O_DIRECT).