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Diffstat (limited to 'doc/kernel/cache-policies.txt')
| -rw-r--r-- | doc/kernel/cache-policies.txt | 38 |
1 files changed, 29 insertions, 9 deletions
diff --git a/doc/kernel/cache-policies.txt b/doc/kernel/cache-policies.txt index d7c440b44..0d124a971 100644 --- a/doc/kernel/cache-policies.txt +++ b/doc/kernel/cache-policies.txt @@ -30,28 +30,48 @@ multiqueue This policy is the default. -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. +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). + 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 <value>' + 'write_promote_adjustment <value>' + 'discard_promote_adjustment <value>' 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. cleaner ------- |