NVIDIA / cuCollections

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[FEATURE]: Add `perfect_hashing` probing scheme #487

Open sleeepyjack opened 4 months ago

sleeepyjack commented 4 months ago

Is your feature request related to a problem? Please describe.

Perfect hash functions describe an injective mapping from the input key domain into the hash map's slot index domain. In other words, Each distinct key hashes to a distinct slot in the map.

This setup allows for a set of optimizations:

Describe the solution you'd like

Add a new class cuco::perfect_hashing<class Hash> to our probing scheme zoo which behaves as follows:

When the dereferencing operator of the probing iterator is called for the first time (at the initial probing position), return slots + hash(key). After incrementing the iterator, always return end(), meaning that there is at most one probing step.

A user must ensure that the Hash function in combination with the input key set actually forms a perfect hash function, and the maximum hash values is smaller than the map's capacity. Otherwise behavior is undefined.

Notes on the implementation:

Describe alternatives you've considered

There is one more optimization we could additionally apply, but I would vote against it due to technical reasons:

Perfect hashing guarantees that there are no collisions. Thus, we could insert keys using non-atomic STG instructions, which has proven to be significantly faster compared to atomic CAS operations. This however leads to some undesireful side effects due to the relaxed memory ordering of the GPU, which ultimately leads to implausible return values from some of our APIs (insert_and_find and also bulk insert; see example in the bottom paragraph of https://github.com/NVIDIA/cuCollections/issues/475#issuecomment-2113437463).

If this optimization is desired, it can still be enabled by specifying cuda::thread_scope_thread when instantiating the map type. This is a bit hacky but I think it's better than breaking the existing logic, introducing spurious errors in the aforementioned return values.

Additional context

See discussion #475

### Tasks
sleeepyjack commented 4 months ago

Few more implementation details come to my mind:

njones93531 commented 3 months ago

As mentioned in slack, I would like to work on this issue