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authorJoe Thornber <ejt@redhat.com>2015-05-15 15:33:34 +0100
committerMike Snitzer <snitzer@redhat.com>2015-06-11 17:12:59 -0400
commit66a636356647a9be8885c2ce2948de126577698a (patch)
tree237f4256a6b10a14c75174b8ffe2729803e3e72c /drivers/md/Kconfig
parent40775257b97e27305cf5c2425be7acaa6edee4ea (diff)
dm cache: add stochastic-multi-queue (smq) policy
The stochastic-multi-queue (smq) policy addresses some of the problems with the current multiqueue (mq) policy. 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 ----------- In my tests SMQ shows substantially better performance than MQ. Once this matures a bit more I'm sure it'll become the default policy. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Diffstat (limited to 'drivers/md/Kconfig')
-rw-r--r--drivers/md/Kconfig12
1 files changed, 12 insertions, 0 deletions
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index edcf4ab66e00..b59727309072 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -304,6 +304,18 @@ config DM_CACHE_MQ
This is meant to be a general purpose policy. It prioritises
reads over writes.
+config DM_CACHE_SMQ
+ tristate "Stochastic MQ Cache Policy (EXPERIMENTAL)"
+ depends on DM_CACHE
+ default y
+ ---help---
+ A cache policy that uses a multiqueue ordered by recent hits
+ to select which blocks should be promoted and demoted.
+ This is meant to be a general purpose policy. It prioritises
+ reads over writes. This SMQ policy (vs MQ) offers the promise
+ of less memory utilization, improved performance and increased
+ adaptability in the face of changing workloads.
+
config DM_CACHE_CLEANER
tristate "Cleaner Cache Policy (EXPERIMENTAL)"
depends on DM_CACHE