[jira] [Created] (KAFKA-5203) Percentilles are calculated incorrectly

["Ivan A. Melnikov (JIRA)" <ji...@apache.org>]

Ivan A. Melnikov created KAFKA-5203:
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             Summary: Percentilles are calculated incorrectly
                 Key: KAFKA-5203
                 URL: https://issues.apache.org/jira/browse/KAFKA-5203
             Project: Kafka
          Issue Type: Bug
          Components: metrics
            Reporter: Ivan A. Melnikov
            Priority: Minor


After the samples are purged couple of times, the calculated percentile values tend to decrease comparing to the expected values.

Consider the following simple example (sorry, idk if I can make it shorter):

{code}
        int buckets = 100;
        Metrics metrics = new Metrics(new MetricConfig().eventWindow(buckets/2).samples(2));
        Sensor sensor = metrics.sensor("test");
        sensor.add(new Percentiles(4 * buckets, 100.0, Percentiles.BucketSizing.CONSTANT,
                new Percentile(metrics.metricName("test.p50", "grp1"), 50),
                new Percentile(metrics.metricName("test.p75", "grp1"), 75)));
        Metric p50 = metrics.metrics().get(metrics.metricName("test.p50", "grp1"));
        Metric p75 = metrics.metrics().get(metrics.metricName("test.p75", "grp1"));

        for (int i = 0; i < buckets; i++) sensor.record(i);
        System.out.printf("p50=%.3f p75=%.3f\n", p50.value(), p75.value());
        for (int i = 0; i < buckets; i++) sensor.record(i);
        System.out.printf("p50=%.3f p75=%.3f\n", p50.value(), p75.value());
        for (int i = 0; i < buckets; i++) sensor.record(i);
        System.out.printf("p50=%.3f p75=%.3f\n", p50.value(), p75.value());
{code}

The output from this is:
{noformat}
p50=50.000 p75=74.490
p50=24.490 p75=36.735
p50=15.306 p75=24.490
{noformat}

The expected output is, of course, with all three lines similar to the first one.



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