--- zzzz-none-000/linux-3.10.107/Documentation/device-mapper/cache-policies.txt 2017-06-27 09:49:32.000000000 +0000 +++ scorpion-7490-727/linux-3.10.107/Documentation/device-mapper/cache-policies.txt 2021-02-04 17:41:59.000000000 +0000 @@ -25,33 +25,114 @@ 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 +clean entries and a set for dirty entries). -The multiqueue policy has two sets of 16 queues: one set for entries -waiting for the cache and another one for those in the cache. Cache entries in the queues are aged based on logical time. Entry into the cache is based on variable thresholds and queue selection is based on hit count on entry. The policy aims to take different cache miss costs into account and to adjust to varying load patterns automatically. Message and constructor argument pairs are: - 'sequential_threshold <#nr_sequential_ios>' and - 'random_threshold <#nr_random_ios>'. + 'sequential_threshold <#nr_sequential_ios>' + 'random_threshold <#nr_random_ios>' + 'read_promote_adjustment ' + 'write_promote_adjustment ' + 'discard_promote_adjustment ' The sequential threshold indicates the number of contiguous I/Os -required before a stream is treated as sequential. The random threshold +required before a stream is treated as sequential. Once a stream is +considered sequential it will bypass the cache. The random threshold is the number of intervening non-contiguous I/Os that must be seen before the stream is treated as random again. The sequential and random thresholds default to 512 and 4 respectively. -Large, sequential ios are probably better left on the origin device -since spindles tend to have good bandwidth. The io_tracker counts -contiguous I/Os to try to spot when the io is in one of these sequential -modes. +Large, sequential I/Os are probably better left on the origin device +since spindles tend to have good sequential I/O bandwidth. The +io_tracker counts contiguous I/Os to try to spot when the I/O is in one +of these sequential modes. But there are use-cases for wanting to +promote sequential blocks to the cache (e.g. fast application startup). +If sequential threshold is set to 0 the sequential I/O detection is +disabled and sequential I/O will no longer implicitly bypass the cache. +Setting the random threshold to 0 does _not_ disable the random I/O +stream detection. + +Internally the mq policy determines a promotion threshold. If the hit +count of a block not in the cache goes above this threshold it gets +promoted to the cache. The read, write and discard promote adjustment +tunables allow you to tweak the promotion threshold by adding a small +value based on the io type. They default to 4, 8 and 1 respectively. +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 -------