Janus Chow created HDFS-17931:
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Summary: Add early return in
PendingDataNodeMessages.removeQueuedBlock when queue is empty
Key: HDFS-17931
URL: https://issues.apache.org/jira/browse/HDFS-17931
Project: Hadoop HDFS
Issue Type: Improvement
Reporter: Janus Chow
Assignee: Janus Chow
removeQueuedBlock() is called for every block in a block report. When the
pending queue is empty (count == 0), the method still unconditionally allocates
a new Block() and does a HashMap.get() before returning null. On large
DataNodes this is millions of unnecessary allocations per report.
This is especially costly under ZGC, where these operations trigger load/write
barriers even on the no-op path. In practice, on a ZGC Observer NameNode, count
is almost always 0 because ZGC's sub-millisecond pauses keep EditLogTailer
current, so isGenStampInFuture() rarely triggers.
Fix: add a count == 0 early return before the new Block() allocation.
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