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From "Peter Schuller (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-2466) bloom filters should avoid huge array allocations to avoid fragmentation concerns
Date Wed, 13 Apr 2011 03:46:05 GMT
bloom filters should avoid huge array allocations to avoid fragmentation concerns
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                 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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