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https://issues.apache.org/jira/browse/CASSANDRA-2466?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13019457#comment-13019457
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Ryan King commented on CASSANDRA-2466:
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Moving to smaller arrays would make the allocation easier, but wouldn't reduce
the raw amount of memory needed for a large bloom filter.
Would it be worth moving these off-heap completely?
> bloom filters should avoid huge array allocations to avoid fragmentation
> concerns
> ---------------------------------------------------------------------------------
>
> Key: CASSANDRA-2466
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2466
> Project: Cassandra
> Issue Type: Bug
> Reporter: Peter Schuller
> Priority: Minor
>
> The fact that bloom filters are backed by single large arrays of longs is
> expected to interact badly with promotion of objects into old gen with CMS,
> due to fragmentation concerns (as discussed in CASSANDRA-2463).
> It should be less of an issue than CASSANDRA-2463 in the sense that you need
> to have a lot of rows before the array sizes become truly huge. For
> comparison, the ~ 143 million row key limit implied by the use of 'int' in
> BitSet prior to the switch to OpenBitSet translates roughly to 238 MB
> (assuming the limitation factor there was the addressability of the bits with
> a 32 bit int, which is my understanding).
> Having a preliminary look at OpenBitSet with an eye towards replacing the
> single long[] with multiple arrays, it seems that if we're willing to drop
> some of the functionality that is not used for bloom filter purposes, the
> bits[i] indexing should be pretty easy to augment with modulo to address an
> appropriate smaller array. Locality is not an issue since the bloom filter
> case is the worst possible case for locality anyway, and it doesn't matter
> whether it's one huge array or a number of ~ 64k arrays.
> Callers may be affected like BloomFilterSerializer which cares about the
> underlying bit array.
> If the full functionality of OpenBitSet is to be maintained (e.g., xorCount)
> some additional acrobatics would be necessary and presumably at a noticable
> performance cost if such operations were to be used in performance critical
> places.
> An argument against touching OpenBitSet is that it seems to be pretty
> carefully written and tested and has some non-trivial details and people have
> seemingly benchmarked it quite carefully. On the other hand, the improvement
> would then apply to other things as well, such as the bitsets used to keep
> track of in-core pages (off the cuff for scale, a 64 gig sstable should imply
> a 2 mb bit set, with one bit per 4k page).
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