Commit a08d8b0
[SPARK-47547][CORE] Add
### What changes were proposed in this pull request?
This change fixes a performance degradation issue in the current BloomFilter implementation.
The current bit index calculation logic does not use any part of the indexable space above the first 31bits, so when the inserted item count approaches (or exceeds) Integer.MAX_VALUE, it will produce significantly worse collision rates than an (ideal) uniformly distributing hash function.
### Why are the changes needed?
This should qualify as a bug.
The upper bound on the bit capacity of the current BloomFilter implementation in spark is approx 137G bits (64 bit longs in an Integer.MAX_VALUE sized array). The current indexing scheme can only address about 2G bits of these.
On the other hand, due to the way the BloomFilters are used, the bug won't cause any logical errors, it will gradually render the BloomFilter instance useless by forcing more-and-more queries on the slow path.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
#### new test
One new java testclass was added to `sketch` to test different combinations of item counts and expected fpp rates.
```
common/sketch/src/test/java/org/apache/spark/util/sketch/TestSparkBloomFilter.java
```
`testAccuracyEvenOdd`
in N number of iterations inserts N even numbers (2\*i), and leaves out N odd numbers (2\*i+1) from the BloomFilter.
The test checks the 100% accuracy of `mightContain=true` on all of the even items, and measures the `mightContain=true` (false positive) rate on the not-inserted odd numbers.
`testAccuracyRandom`
in 2N number of iterations inserts N pseudorandomly generated numbers in two differently seeded (theoretically independent) BloomFilter instances. All the random numbers generated in an even-iteration will be inserted into both filters, all the random numbers generated in an odd-iteration will be left out from both.
The test checks the 100% accuracy of `mightContain=true` for all of the items inserted in an even-loop. It counts the false positives as the number of odd-loop items for which the primary filter reports `mightContain=true` but secondary reports `mightContain=false`. Since we inserted the same elements into both instances, and the secondary reports non-insertion, the `mightContain=true` from the primary can only be a false positive.
#### patched
One minor (test) issue was fixed in
```
common/sketch/src/test/scala/org/apache/spark/util/sketch/BloomFilterSuite.scala
```
where the potential repetitions in the randomly generated stream of insertable items resulted in slightly worse fpp measurements than the actual. The problem affected the those testcases more where the cardinality of the tested type is low (the chance of repetition is high), e.g. Byte and Short.
### Was this patch authored or co-authored using generative AI tooling?
No
Closes #50933 from ishnagy/SPARK-47547_bloomfilter_fpp_degradation.
Authored-by: Ish Nagy <[email protected]>
Signed-off-by: Peter Toth <[email protected]>BloomFilter V2 and use it as default1 parent 23a19e6 commit a08d8b0
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lines changed- common/sketch/src
- main/java/org/apache/spark/util/sketch
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- java/org/apache/spark/util/sketch
- scala/org/apache/spark/util/sketch
- sql/core/src/test/scala/org/apache/spark/sql
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