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Diffstat (limited to 'Documentation/device-mapper/cache-policies.txt')
| -rw-r--r-- | Documentation/device-mapper/cache-policies.txt | 67 |
1 files changed, 64 insertions, 3 deletions
diff --git a/Documentation/device-mapper/cache-policies.txt b/Documentation/device-mapper/cache-policies.txt index 0d124a971801..d9246a32e673 100644 --- a/Documentation/device-mapper/cache-policies.txt +++ b/Documentation/device-mapper/cache-policies.txt @@ -25,10 +25,10 @@ trying to see when the io scheduler has let the ios run. Overview of supplied cache replacement policies =============================================== -multiqueue ----------- +multiqueue (mq) +--------------- -This policy is the default. +This policy has been deprecated in favor of the smq policy (see below). The multiqueue policy has three sets of 16 queues: one set for entries waiting for the cache and another two for those in the cache (a set for @@ -73,6 +73,67 @@ If you're trying to quickly warm a new cache device you may wish to reduce these to encourage promotion. Remember to switch them back to their defaults after the cache fills though. +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 uses 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 +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 these mean 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 +multiqueue. + +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 +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 +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 +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 +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 ------- |