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author | Linus Torvalds <torvalds@linux-foundation.org> | 2019-07-09 22:34:26 +0300 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2019-07-09 22:34:26 +0300 |
commit | e9a83bd2322035ed9d7dcf35753d3f984d76c6a5 (patch) | |
tree | 66dc466ff9aec0f9bb7f39cba50a47eab6585559 /Documentation/device-mapper/cache-policies.rst | |
parent | 7011b7e1b702cc76f9e969b41d9a95969f2aecaa (diff) | |
parent | 454f96f2b738374da4b0a703b1e2e7aed82c4486 (diff) | |
download | linux-e9a83bd2322035ed9d7dcf35753d3f984d76c6a5.tar.xz |
Merge tag 'docs-5.3' of git://git.lwn.net/linux
Pull Documentation updates from Jonathan Corbet:
"It's been a relatively busy cycle for docs:
- A fair pile of RST conversions, many from Mauro. These create more
than the usual number of simple but annoying merge conflicts with
other trees, unfortunately. He has a lot more of these waiting on
the wings that, I think, will go to you directly later on.
- A new document on how to use merges and rebases in kernel repos,
and one on Spectre vulnerabilities.
- Various improvements to the build system, including automatic
markup of function() references because some people, for reasons I
will never understand, were of the opinion that
:c:func:``function()`` is unattractive and not fun to type.
- We now recommend using sphinx 1.7, but still support back to 1.4.
- Lots of smaller improvements, warning fixes, typo fixes, etc"
* tag 'docs-5.3' of git://git.lwn.net/linux: (129 commits)
docs: automarkup.py: ignore exceptions when seeking for xrefs
docs: Move binderfs to admin-guide
Disable Sphinx SmartyPants in HTML output
doc: RCU callback locks need only _bh, not necessarily _irq
docs: format kernel-parameters -- as code
Doc : doc-guide : Fix a typo
platform: x86: get rid of a non-existent document
Add the RCU docs to the core-api manual
Documentation: RCU: Add TOC tree hooks
Documentation: RCU: Rename txt files to rst
Documentation: RCU: Convert RCU UP systems to reST
Documentation: RCU: Convert RCU linked list to reST
Documentation: RCU: Convert RCU basic concepts to reST
docs: filesystems: Remove uneeded .rst extension on toctables
scripts/sphinx-pre-install: fix out-of-tree build
docs: zh_CN: submitting-drivers.rst: Remove a duplicated Documentation/
Documentation: PGP: update for newer HW devices
Documentation: Add section about CPU vulnerabilities for Spectre
Documentation: platform: Delete x86-laptop-drivers.txt
docs: Note that :c:func: should no longer be used
...
Diffstat (limited to 'Documentation/device-mapper/cache-policies.rst')
-rw-r--r-- | Documentation/device-mapper/cache-policies.rst | 131 |
1 files changed, 131 insertions, 0 deletions
diff --git a/Documentation/device-mapper/cache-policies.rst b/Documentation/device-mapper/cache-policies.rst new file mode 100644 index 000000000000..b17fe352fc41 --- /dev/null +++ b/Documentation/device-mapper/cache-policies.rst @@ -0,0 +1,131 @@ +============================= +Guidance for writing policies +============================= + +Try to keep transactionality out of it. The core is careful to +avoid asking about anything that is migrating. This is a pain, but +makes it easier to write the policies. + +Mappings are loaded into the policy at construction time. + +Every bio that is mapped by the target is referred to the policy. +The policy can return a simple HIT or MISS or issue a migration. + +Currently there's no way for the policy to issue background work, +e.g. to start writing back dirty blocks that are going to be evicted +soon. + +Because we map bios, rather than requests it's easy for the policy +to get fooled by many small bios. For this reason the core target +issues periodic ticks to the policy. It's suggested that the policy +doesn't update states (eg, hit counts) for a block more than once +for each tick. The core ticks by watching bios complete, and so +trying to see when the io scheduler has let the ios run. + + +Overview of supplied cache replacement policies +=============================================== + +multiqueue (mq) +--------------- + +This policy is now an alias for smq (see below). + +The following tunables are accepted, but have no effect:: + + 'sequential_threshold <#nr_sequential_ios>' + 'random_threshold <#nr_random_ios>' + 'read_promote_adjustment <value>' + 'write_promote_adjustment <value>' + 'discard_promote_adjustment <value>' + +Stochastic multiqueue (smq) +--------------------------- + +This policy is the default. + +The stochastic multi-queue (smq) policy addresses some of the problems +with the multiqueue (mq) policy. + +The smq policy (vs mq) offers the promise of less memory utilization, +improved performance and increased adaptability in the face of changing +workloads. smq also does not have any cumbersome tuning knobs. + +Users may switch from "mq" to "smq" simply by appropriately reloading a +DM table that is using the cache target. Doing so will cause all of the +mq policy's hints to be dropped. Also, performance of the cache may +degrade slightly until smq recalculates the origin device's hotspots +that should be cached. + +Memory usage +^^^^^^^^^^^^ + +The mq policy used a lot of memory; 88 bytes per cache block on a 64 +bit machine. + +smq uses 28bit indexes to implement its data structures rather than +pointers. It avoids storing an explicit hit count for each block. It +has a 'hotspot' queue, rather than a pre-cache, which uses a quarter of +the entries (each hotspot block covers a larger area than a single +cache block). + +All this means smq uses ~25bytes per cache block. Still a lot of +memory, but a substantial improvement nontheless. + +Level balancing +^^^^^^^^^^^^^^^ + +mq placed entries in different levels of the multiqueue structures +based on their hit count (~ln(hit count)). This meant the bottom +levels generally had the most entries, and the top ones had very +few. Having unbalanced levels like this reduced the efficacy of the +multiqueue. + +smq does not maintain a hit count, instead it swaps hit entries with +the least recently used entry from the level above. The overall +ordering being a side effect of this stochastic process. With this +scheme we can decide how many entries occupy each multiqueue level, +resulting in better promotion/demotion decisions. + +Adaptability: +The mq policy maintained a hit count for each cache block. For a +different block to get promoted to the cache its hit count has to +exceed the lowest currently in the cache. This meant it could take a +long time for the cache to adapt between varying IO patterns. + +smq doesn't maintain hit counts, so a lot of this problem just goes +away. In addition it tracks performance of the hotspot queue, which +is used to decide which blocks to promote. If the hotspot queue is +performing badly then it starts moving entries more quickly between +levels. This lets it adapt to new IO patterns very quickly. + +Performance +^^^^^^^^^^^ + +Testing smq shows substantially better performance than mq. + +cleaner +------- + +The cleaner writes back all dirty blocks in a cache to decommission it. + +Examples +======== + +The syntax for a table is:: + + cache <metadata dev> <cache dev> <origin dev> <block size> + <#feature_args> [<feature arg>]* + <policy> <#policy_args> [<policy arg>]* + +The syntax to send a message using the dmsetup command is:: + + dmsetup message <mapped device> 0 sequential_threshold 1024 + dmsetup message <mapped device> 0 random_threshold 8 + +Using dmsetup:: + + dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \ + /dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8" + creates a 128GB large mapped device named 'blah' with the + sequential threshold set to 1024 and the random_threshold set to 8. |