[jira] [Created] (CASSANDRA-15834) Bloom filter false positive rate calculation does not take into account true negatives

["Jaroslaw Grabowski (Jira)" <ji...@apache.org>]

Jaroslaw Grabowski created CASSANDRA-15834:
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             Summary: Bloom filter false positive rate calculation does not take into account true negatives
                 Key: CASSANDRA-15834
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-15834
             Project: Cassandra
          Issue Type: Bug
            Reporter: Jaroslaw Grabowski


The bloom filter false positive ratio is [currently computed|https://github.com/apache/cassandra/blob/ded62076e7fdfd1cfdcf96447489ea607ca796a0/src/java/org/apache/cassandra/metrics/TableMetrics.java#L738] as:

{\{bf_fp_ratio = false_positive_count / (false_positive_count + true_positive_count)}}

However, this calculation doesn't take into account true negatives (false negatives never happen on bloom filters).

In a situation where there are 1000 reads for non existing rows, and there are 10 false positives, the bloom filter false positive ratio will be wrongly calculated as 10/10 = 1.0, while it should be 10/1000 = 0.01.

We should update the calculation to:

{\{bf_fp_ratio = false_positive_count / #bf_queries}}



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