[KAFKA-5203] Percentilles are calculated incorrectly

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 0.11.0.0
• Component/s: metrics
• Assignee: Unassigned
• Reporter: Ivan A. Melnikov
• Created: 09/May/17 12:14
• Updated: 15/May/17 17:52
• Resolved: 15/May/17 17:51

Description

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):

        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());

The output from this is:

p50=50.000 p75=74.490
p50=24.490 p75=36.735
p50=15.306 p75=24.490

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

Issue Links

• links to: GitHub Pull Request #3002