leerho edited a comment on issue #6865: Densify swapped hll buffer URL: https://github.com/apache/incubator-druid/pull/6865#issuecomment-461680212 @drcrallen I have several comments on your situation: - First of all, you are managing your own hash function that feeds the HLLC sketch. This is a No No! I had mentioned this to @gianm in #6814 recently and he assured me: > This isn't true in practice. The druid-hll library lets callers use any hash function, but Druid doesn't expose that to end users. It always uses `Hashing.murmur3_128()` The sketch must do its own hashing preferably with its own hash function and with a private seed and users should not peek inside and use the same hash function with the same seed for performing an upstream modulo sampling as you do in `testCanFillUpOnMod()`. HLL sketches are stochastic functions that rely on good randomness properties of the hash function that are **independent** of the incoming data! So by using the same exact hash function and the same seed in your mod function you are violating this independence property and all bets are off! - Nonetheless, what you also have uncovered is likely a bug. I took your test and added the DataSketches HLL sketch in parallel inside the test. I also added a few more outputs at the end and got these results: ``` Filled up registers after 3,918,870 random numbers Count: 19590 HLLc Uniques: 0 DS-HLL Uniques: 19169.299781 ``` The Count is the number of times a value is added to the sketch (at the bottom of the do loop). This is not the true number of uniques as there may be a few collisions amongst those 4M random numbers, but it was adequate for this experiment. The Druid HLL shows a count of zero. I did not debug this but perhaps by checking the NumNonZeroRegisters variable just when it hits zero, you are catching the sketch just before it transitions. I am not sure, I am kinda surprised by this. The DS-HLL shows a count of 19169 which is well within the error bounds of the sketch of that size. *** As for your suggested change, I'm really not sure what ultimate effect it will have. Cheers
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