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dm cache: update cache-policies.txt now that mq is an alias for smq 

Also fix some typos and make all "smq" and "mq" references consistently
lowercase.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
This commit is contained in:
Mike Snitzer 2016-04-20 21:11:25 -04:00
parent 2da1610ae2
commit 492d48db8d
1 changed files with 16 additions and 18 deletions
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34
Documentation/device-mapper/cache-policies.txt
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@ -11,7 +11,7 @@ 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. 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, 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 evicte e.g. to start writing back dirty blocks that are going to be evicted
soon. soon.
Because we map bios, rather than requests it's easy for the policy Because we map bios, rather than requests it's easy for the policy
@ -48,7 +48,7 @@ with the multiqueue (mq) policy.
The smq policy (vs mq) offers the promise of less memory utilization, The smq policy (vs mq) offers the promise of less memory utilization,
improved performance and increased adaptability in the face of changing improved performance and increased adaptability in the face of changing
workloads. SMQ also does not have any cumbersome tuning knobs. workloads. smq also does not have any cumbersome tuning knobs.
Users may switch from "mq" to "smq" simply by appropriately reloading a 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 DM table that is using the cache target. Doing so will cause all of the
@ -57,47 +57,45 @@ degrade slightly until smq recalculates the origin device's hotspots
that should be cached. that should be cached.
Memory usage: Memory usage:
The mq policy uses a lot of memory; 88 bytes per cache block on a 64 The mq policy used a lot of memory; 88 bytes per cache block on a 64
bit machine. bit machine.
SMQ uses 28bit indexes to implement it's data structures rather than smq uses 28bit indexes to implement it's data structures rather than
pointers. It avoids storing an explicit hit count for each block. It 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 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 the entries (each hotspot block covers a larger area than a single
cache block). cache block).
All these mean smq uses ~25bytes per cache block. Still a lot of All this means smq uses ~25bytes per cache block. Still a lot of
memory, but a substantial improvement nontheless. memory, but a substantial improvement nontheless.
Level balancing: Level balancing:
MQ places entries in different levels of the multiqueue structures mq placed entries in different levels of the multiqueue structures
based on their hit count (~ln(hit count)). This means the bottom based on their hit count (~ln(hit count)). This meant the bottom
levels generally have the most entries, and the top ones have very levels generally had the most entries, and the top ones had very
few. Having unbalanced levels like this reduces the efficacy of the few. Having unbalanced levels like this reduced the efficacy of the
multiqueue. multiqueue.
SMQ does not maintain a hit count, instead it swaps hit entries with smq does not maintain a hit count, instead it swaps hit entries with
the least recently used entry from the level above. The over all the least recently used entry from the level above. The overall
ordering being a side effect of this stochastic process. With this ordering being a side effect of this stochastic process. With this
scheme we can decide how many entries occupy each multiqueue level, scheme we can decide how many entries occupy each multiqueue level,
resulting in better promotion/demotion decisions. resulting in better promotion/demotion decisions.
Adaptability: Adaptability:
The MQ policy maintains a hit count for each cache block. For a The mq policy maintained a hit count for each cache block. For a
different block to get promoted to the cache it's hit count has to different block to get promoted to the cache it's hit count has to
exceed the lowest currently in the cache. This means it can take a exceed the lowest currently in the cache. This meant it could take a
long time for the cache to adapt between varying IO patterns. long time for the cache to adapt between varying IO patterns.
Periodically degrading the hit counts could help with this, but I
haven't found a nice general solution.
SMQ doesn't maintain hit counts, so a lot of this problem just goes 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 away. In addition it tracks performance of the hotspot queue, which
is used to decide which blocks to promote. If the hotspot queue is is used to decide which blocks to promote. If the hotspot queue is
performing badly then it starts moving entries more quickly between performing badly then it starts moving entries more quickly between
levels. This lets it adapt to new IO patterns very quickly. levels. This lets it adapt to new IO patterns very quickly.
Performance: Performance:
Testing SMQ shows substantially better performance than MQ. Testing smq shows substantially better performance than mq.
cleaner cleaner
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