[CASSANDRA-14085] Excessive update of ReadLatency metric in digest calculation

• Type: Bug
• Status: Resolved
• Priority: Low
• Resolution: Not A Problem
• Fix Version/s: 3.0.26, 3.11.12, 4.0.1, 4.1-alpha1, 4.1
• Component/s: Legacy/Core, Observability/Metrics
• Severity: Low
• Since Version: 3.0.0
• Assignee: Andrew Whang
• Reporter: Andrew Whang

Authors

Andrew Whang

• Created: 30/Nov/17 22:27
• Updated: 16/Dec/22 21:56
• Resolved: 30/Jun/21 15:15

Description

We noticed an increase in read latency after upgrading to 3.x, specifically for requests with CL>ONE. It turns out the read latency metric is being doubly updated for digest calculations. This code (https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/db/partitions/UnfilteredPartitionIterators.java#L243) makes an improper copy of an iterator that's wrapped by MetricRecording, whose onClose() records the latency of the execution.