[jira] [Created] (KAFKA-14664) Raft idle ratio is inaccurate

["Jason Gustafson (Jira)" <ji...@apache.org>]

Jason Gustafson created KAFKA-14664:
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             Summary: Raft idle ratio is inaccurate
                 Key: KAFKA-14664
                 URL: https://issues.apache.org/jira/browse/KAFKA-14664
             Project: Kafka
          Issue Type: Bug
            Reporter: Jason Gustafson
            Assignee: Jason Gustafson


The `poll-idle-ratio-avg` metric is intended to track how idle the raft IO thread is. When completely idle, it should measure 1. When saturated, it should measure 0. The problem with the current measurements is that they are treated equally with respect to time. For example, say we poll twice with the following durations:

Poll 1: 2s

Poll 2: 0s

Assume that the busy time is negligible, so 2s passes overall.

In the first measurement, 2s is spent waiting, so we compute and record a ratio of 1.0. In the second measurement, no time passes, and we record 0.0. The idle ratio is then computed as the average of these two values (1.0 + 0.0 / 2 = 0.5), which suggests that the process was busy for 1s. 

Instead, we should sum up the time waiting over the full interval. 2s passes total here and 2s is idle, so we should compute 1.0.



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