Andrei Dulceanu created OAK-7262:
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Summary: LockBasedScheduler#getHeadNodeState poor performance due
to lock contention in commitTimeHistogram implementation
Key: OAK-7262
URL: https://issues.apache.org/jira/browse/OAK-7262
Project: Jackrabbit Oak
Issue Type: Improvement
Components: segment-tar
Reporter: Andrei Dulceanu
Assignee: Andrei Dulceanu
Fix For: 1.9.0, 1.10
With OAK-4732, we slightly prioritised reads, allowing callers of
{{LockBasedScheduler#getHeadNodeState}} to wait for a certain amount of time on
the commit semaphore, in order to fetch the latest head. The amount of time to
wait for was computed as the median of the latest 1000 commits, but the
implementation was flexible enough to replace the 0.5 quantile with a different
value.
The initial implementation used {{SynchronizedDescriptiveStatistics}} from
{{commons-math3}}. In OAK-6430, we replaced that with a {{Histogram}} from the
dropwizard metric library (\{{io.dropwizard.metrics}}).
One of the problems observed with the current implementation is that, under
heavy loading, the performance of {{LockBasedScheduler#getHeadNodeState}}
decreases due to lock contention in {{commitTimeHistogram}} implementation.
Namely, {{SlidingWindowReservoir#getSnapshot}} seems to be a bottleneck. It
copies an array of 1000 elements at each invocation of
{{LockBasedScheduler#getHeadNodeState}}, acquiring and releasing a lock for
each element in the array.
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