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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