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https://issues.apache.org/jira/browse/CASSANDRA-1220?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12898133#action_12898133
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Jeff Hodges commented on CASSANDRA-1220:
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This is a nice change.
> Improve Bloom filter efficiency
> -------------------------------
>
> Key: CASSANDRA-1220
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1220
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Coda Hale
> Priority: Minor
> Attachments: bloom-filter-calculation-correction.patch
>
>
> Attached is a patch which resolves two issues:
> First, the table of false positives for a given number of bits per element
> and a given number of hashes in {{BloomCalculations}} is incomplete. This
> patch includes the calculations from Table 5 of [Bloom Filters - The
> Math|http://pages.cs.wisc.edu/~cao/papers/summary-cache/node8.html#327],
> which is referenced in the documentation at the source of the table. This
> changes the optimal number of hashes for the Bloom filters created by
> {{SSTableWriter}} (with 15 bits per element) from 8 to 10, with a reduction
> in the false positive rate from 0.000852 to 0.000744. (The patch also extends
> the table to include false positive rates for 16-20 bits per element for
> future reference.)
> Second, the values of {{BloomCalculations.optKPerBuckets}} were not correct,
> in that some of them did not choose the optimal number of hashes for a given
> number of bits per element, probably due to transcription errors in the past.
> This patch replaces that series of constants with an array of optimal hashes
> generated during {{BloomCalculation}}'s static initialization.
> This patch should apply cleanly to the {{0.6}} branch and {{trunk}} with all
> tests passing.
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