It was just an oversight. Thinking about it now, u64 could have been better:
1. intra element collison goes down: 1/32 = 3.1% vs 1/64 = 1.6% -> 1.5% difference. Intra element collison doesn't mean guaranteed a FP though!
2. 64 bucket would have less variance - filter behavior would be more predictable
But there is a downside:
When switching from 32 to 64 bit word we also reduce number of array elements 2 times, doubling the contention. We are populating the filter from up to 128 threads during join phase. When the build side isn't significantly smaller than the probe side, that contention can overweight the FP improvement.
Comments
It was just an oversight. Thinking about it now, u64 could have been better:
1. intra element collison goes down: 1/32 = 3.1% vs 1/64 = 1.6% -> 1.5% difference. Intra element collison doesn't mean guaranteed a FP though! 2. 64 bucket would have less variance - filter behavior would be more predictable
But there is a downside: When switching from 32 to 64 bit word we also reduce number of array elements 2 times, doubling the contention. We are populating the filter from up to 128 threads during join phase. When the build side isn't significantly smaller than the probe side, that contention can overweight the FP improvement.