diff options
| -rw-r--r-- | Documentation/device-mapper/cache-policies.txt | 67 | ||||
| -rw-r--r-- | drivers/md/dm-cache-policy-mq.c | 34 | ||||
| -rw-r--r-- | drivers/md/dm-cache-policy-smq.c | 17 |
3 files changed, 86 insertions, 32 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 ------- diff --git a/drivers/md/dm-cache-policy-mq.c b/drivers/md/dm-cache-policy-mq.c index 838665bb495a..32814371b8d3 100644 --- a/drivers/md/dm-cache-policy-mq.c +++ b/drivers/md/dm-cache-policy-mq.c @@ -1431,21 +1431,12 @@ bad_pre_cache_init: static struct dm_cache_policy_type mq_policy_type = { .name = "mq", - .version = {1, 3, 0}, + .version = {1, 4, 0}, .hint_size = 4, .owner = THIS_MODULE, .create = mq_create }; -static struct dm_cache_policy_type default_policy_type = { - .name = "default", - .version = {1, 3, 0}, - .hint_size = 4, - .owner = THIS_MODULE, - .create = mq_create, - .real = &mq_policy_type -}; - static int __init mq_init(void) { int r; @@ -1455,36 +1446,21 @@ static int __init mq_init(void) __alignof__(struct entry), 0, NULL); if (!mq_entry_cache) - goto bad; + return -ENOMEM; r = dm_cache_policy_register(&mq_policy_type); if (r) { DMERR("register failed %d", r); - goto bad_register_mq; - } - - r = dm_cache_policy_register(&default_policy_type); - if (!r) { - DMINFO("version %u.%u.%u loaded", - mq_policy_type.version[0], - mq_policy_type.version[1], - mq_policy_type.version[2]); - return 0; + kmem_cache_destroy(mq_entry_cache); + return -ENOMEM; } - DMERR("register failed (as default) %d", r); - - dm_cache_policy_unregister(&mq_policy_type); -bad_register_mq: - kmem_cache_destroy(mq_entry_cache); -bad: - return -ENOMEM; + return 0; } static void __exit mq_exit(void) { dm_cache_policy_unregister(&mq_policy_type); - dm_cache_policy_unregister(&default_policy_type); kmem_cache_destroy(mq_entry_cache); } diff --git a/drivers/md/dm-cache-policy-smq.c b/drivers/md/dm-cache-policy-smq.c index 66feb307e697..80f02d3330e2 100644 --- a/drivers/md/dm-cache-policy-smq.c +++ b/drivers/md/dm-cache-policy-smq.c @@ -1748,6 +1748,15 @@ static struct dm_cache_policy_type smq_policy_type = { .create = smq_create }; +static struct dm_cache_policy_type default_policy_type = { + .name = "default", + .version = {1, 0, 0}, + .hint_size = 4, + .owner = THIS_MODULE, + .create = smq_create, + .real = &smq_policy_type +}; + static int __init smq_init(void) { int r; @@ -1758,12 +1767,20 @@ static int __init smq_init(void) return -ENOMEM; } + r = dm_cache_policy_register(&default_policy_type); + if (r) { + DMERR("register failed (as default) %d", r); + dm_cache_policy_unregister(&smq_policy_type); + return -ENOMEM; + } + return 0; } static void __exit smq_exit(void) { dm_cache_policy_unregister(&smq_policy_type); + dm_cache_policy_unregister(&default_policy_type); } module_init(smq_init); |