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From "Carl Yeksigian (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-8883) Percentile computation should use ceil not floor in EstimatedHistogram
Date Mon, 23 Nov 2015 21:24:11 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-8883?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Carl Yeksigian updated CASSANDRA-8883:
--------------------------------------
    Component/s: Observability

> Percentile computation should use ceil not floor in EstimatedHistogram
> ----------------------------------------------------------------------
>
>                 Key: CASSANDRA-8883
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8883
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Observability
>            Reporter: Chris Lohfink
>            Assignee: Carl Yeksigian
>            Priority: Minor
>             Fix For: 2.1.5
>
>         Attachments: 8883-2.1.txt
>
>
> When computing the pcount Cassandra uses floor and the comparison with elements is >=
so given a simple example of there being a total of five elements
> {code}
> // data
> [1, 1, 1, 1, 1]
> // offsets
> [1, 2, 3, 4, 5]
> {code}
> Cassandra  would report the 50th percentile as 2.  While 3 is the more expected value.
 As a comparison using numpy
> {code}
> import numpy as np
> np.percentile(np.array([1, 2, 3, 4, 5]), 50)
> ==> 3.0
> {code}
> The percentiles was added in CASSANDRA-4022 but is now used a lot in metrics Cassandra
reports.  I think it should error on the side on overestimating instead of underestimating.




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