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https://issues.apache.org/jira/browse/CASSANDRA-15834?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Jaroslaw Grabowski updated CASSANDRA-15834:
-------------------------------------------
Description:
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}}
Original jira by [~pauloricardomg]
was:
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}}
> 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
> Priority: Normal
>
> 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}}
> Original jira by [~pauloricardomg]
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