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

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 evicte

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

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.

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

that should be cached.

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.

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

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

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.

Level balancing:

MQ places entries in different levels of the multiqueue structures

based on their hit count (~ln(hit count)).  This means the bottom

levels generally have the most entries, and the top ones have very

few.  Having unbalanced levels like this reduces the efficacy of the

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

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 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

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.

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

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.

Testing smq shows substantially better performance than mq.

cleaner

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