Reduce SystemKeyspace truncation record contention #559
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JFR profiles show significant synchronization contention on SystemKeyspace#getTruncationRecord(UUID) as part of committing PaxosState. The SystemKeyspace truncationRecords map is a lazily initialized unmodifiable snapshot of the local node's truncated_at map. We now guard this with a read/write lock to optimistically allow concurrent reads, while falling back to most costly write lock and re-reading the local node's truncated_at from system tables when there are truncation modifications. Contention back trace: org.apache.cassandra.db.SystemKeyspace.getTruncationRecord(UUID):494 org.apache.cassandra.db.SystemKeyspace.getTruncatedAt(UUID):488 org.apache.cassandra.service.paxos.PaxosState.commit(Commit):144 org.apache.cassandra.service.paxos.CommitVerbHandler.doVerb(MessageIn, int):34 org.apache.cassandra.net.MessageDeliveryTask.run():70 java.util.concurrent.Executors$RunnableAdapter.call():539 org.apache.cassandra.concurrent.AbstractLocalAwareExecutorService$FutureTask.run():164 org.apache.cassandra.concurrent.AbstractLocalAwareExecutorService$LocalSessionFutureTask.run():136 org.apache.cassandra.concurrent.SEPWorker.run():110
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JFR profiles show significant synchronization contention on SystemKeyspace#getTruncationRecord(UUID) as part of committing PaxosState.
The SystemKeyspace truncationRecords map is a lazily initialized unmodifiable snapshot of the local node's truncated_at map. We now guard this with a read/write lock to optimistically allow concurrent reads, while falling back to most costly write lock and re-reading the local node's truncated_at from system tables when there are truncation modifications.
Contention back trace: