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From "Rafi Shachar (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-14324) MetricSampleQuantiles maintains quantiles summary in a wrong way
Date Thu, 27 Aug 2015 09:38:45 GMT
Rafi Shachar created HBASE-14324:

             Summary: MetricSampleQuantiles maintains quantiles summary in a wrong way
                 Key: HBASE-14324
                 URL: https://issues.apache.org/jira/browse/HBASE-14324
             Project: HBase
          Issue Type: Bug
          Components: metrics
            Reporter: Rafi Shachar

The MetricSampleQuantiles computes quantiles estimations for data stream according to a paper
published by CKMS in 2005. However the implementation is incorrect compared to the paper.
In particular, the insert and compact methods use the rank of an item in the summary whereas
the rank should be the estimated rank in the stream. Due to this bug the resulting summary
is much larger than required: according to my experiments it is more than 10 times larger.
Furthermore, the summary size is not stable while the distribution doesn't change. When the
number of items continue to grow to the tens of millions the summary size is about 200K while
it should be in the range of few thousands. This has significant on performance. I didn't
see significant effect on accuracy.

In insert batch and compress methods, call allowableError() passing the rank of the item in
the summary. It actually should pass the estimated rank in the stream. In the CKMS paper this
rank is denoted by r_i, which more precisely is the estimated rank of item i-1. allowableError
now considers the size of the summary where it should consider the number of items that has
been observed.

In addition, the class currently uses a static sized buffer of size 500. This yields poor
runtime performance. Increasing this buffer to size 10000 or more yields much better performance
(I'm using it with buffer size of 100K). The buffer size can be dynamic and grow with number
of items in the stream.

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