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https://issues.apache.org/jira/browse/CASSANDRA-14898?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17433131#comment-17433131
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Venkata Harikrishna Nukala commented on CASSANDRA-14898:
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[~jolynch] I had updated the patch for trunk & here is the pr:
[https://github.com/apache/cassandra/pull/1287] & CI is here:
https://app.circleci.com/pipelines/github/nvharikrishna/cassandra/41/workflows/7fb33758-2a6d-464a-bcc5-8b5709d1d997
# I did not made changes to implement AutoCloseable as closing the resource
generally gives the impression that resource it not usable any more but
CacheSerializer later used for serialising too. Using normal method to clear up
the map.
# Added few tests to KeyCacheTest as it has more tests related to key cache.
# Added limit on cache load time which is configurable through
max_cache_load_time param (defaults to 30 sec).
Sorry for the delay. Would appreciate your review on the code changes. Please
give me a day or two for making changes to other branches.
> Key cache loading is very slow when there are many SSTables
> -----------------------------------------------------------
>
> Key: CASSANDRA-14898
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14898
> Project: Cassandra
> Issue Type: Bug
> Components: Legacy/Local Write-Read Paths
> Environment: AWS i3.2xlarge, 4 physical cores (8 threads), 60GB of
> RAM, loading about 8MB of KeyCache with 10k keys in it.
> Reporter: Joey Lynch
> Assignee: Venkata Harikrishna Nukala
> Priority: Low
> Labels: Performance, low-hanging-fruit
> Attachments: key_cache_load_slow.svg
>
>
> While dealing with a production issue today where some 3.0.17 nodes had close
> to ~8k sstables on disk due to excessive write pressure, we had a few nodes
> crash due to OOM and then they took close to 17 minutes to load the key cache
> and recover. This excessive key cache load significantly increased the
> duration of the outage (to mitigate we just removed the saved key cache
> files). For example here is one example taking 17 minutes to load 10k keys,
> or about 10 keys per second (which is ... very slow):
> {noformat}
> INFO [pool-3-thread-1] 2018-11-15 21:50:21,885 AutoSavingCache.java:190 -
> reading saved cache /mnt/data/cassandra/saved_caches/KeyCache-d.db
> INFO [pool-3-thread-1] 2018-11-15 22:07:16,490 AutoSavingCache.java:166 -
> Completed loading (1014606 ms; 10103 keys) KeyCache cache
> {noformat}
> I've witnessed similar behavior in the past with large LCS clusters, and
> indeed it appears that any time the number of sstables is large, KeyCache
> loading takes a _really_ long time. Today I got a flame graph and I believe
> that I found the issue and I think it's reasonably easy to fix. From what I
> can tell the {{KeyCacheSerializer::deserialize}} [method
> |https://github.com/apache/cassandra/blob/06209037ea56b5a2a49615a99f1542d6ea1b2947/src/java/org/apache/cassandra/service/CacheService.java#L445]
> which is called for every key is linear in the number of sstables due to the
> [call|https://github.com/apache/cassandra/blob/06209037ea56b5a2a49615a99f1542d6ea1b2947/src/java/org/apache/cassandra/service/CacheService.java#L459]
> to {{ColumnFamilyStore::getSSTables}} which ends up calling {{View::select}}
> [here|https://github.com/apache/cassandra/blob/06209037ea56b5a2a49615a99f1542d6ea1b2947/src/java/org/apache/cassandra/db/lifecycle/View.java#L139].
> The {{View::select}} call is linear in the number of sstables and causes a
> _lot_ of {{HashSet}}
> [resizing|https://github.com/apache/cassandra/blob/06209037ea56b5a2a49615a99f1542d6ea1b2947/src/java/org/apache/cassandra/db/lifecycle/View.java#L139]
> when the number of sstables is much greater than 16 (the default size of the
> backing {{HashMap}}).
> As we see in the attached flamegraph we spend 50% of our CPU time in these
> {{getSSTable}} calls, of which 36% is spent adding sstables to the HashSet in
> {{View::select}} and 17% is spent just iterating the sstables in the first
> place. A full 16% of CPU time is spent _just resizing the HashMap_. Then
> another 4% is spend calling {{CacheService::findDesc}} which does [a linear
> search|https://github.com/apache/cassandra/blob/06209037ea56b5a2a49615a99f1542d6ea1b2947/src/java/org/apache/cassandra/service/CacheService.java#L475]
> for the sstable generation.
> I believe that this affects at least Cassandra 3.0.17 and trunk, and could be
> pretty easily fixed by either caching the getSSTables call or at the very
> least pre-sizing the {{HashSet}} in {{View::select}} to be the size of the
> sstables map.
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