Use lower bounds to avoid traversals in MST

[aprokop]

No description provided.

[dalg24]

What happened to the version with the scan you showed me? I thought you said it was faster.

[aprokop]

I thought so too. But I repeated experiments on Summit yesterday, and could not reproduce. Maybe I ran so many experiments that different results got confused in my head. So, the current patch does not actually seem to help on GPU. On CPU it's better than what I showed you for two reasons: a) we don't do parallel_scan, and b) more importantly, radius may have already been updated by another thread that converged, so it even more threads drop out.

[aprokop]

I changed to using lower bound only for Serial. I investigated using it on Nvidia A100 (I used OACISS Saturn, where I get consistent timings; on Perlmutter, the timings from run to run are very inconsistent). The summary is:

• If I use it inside operator() (same as the current version for Serial), it runs ~6+% slower
• If I used pre-filtering, it runs ~2% faster

In short, there is no benefit on GPU, so I disabled it there.

On CPU (AMD EPYC 7763) in Serial it speeds up the MST construction by 25%.

[dalg24]

Kokkos::Serial is undefined when the serial backend is not enabled. It is worrisome that the CI passed...

[aprokop]

@dalg24 Did you mean to push seemingly unrelated stuff in example_callback.cpp?

[dalg24]

@dalg24 Did you mean to push seemingly unrelated stuff in example_callback.cpp?

No. Will fix.

[aprokop]

This does not compile:

arborx/src/details/ArborX_MinimumSpanningTree.hpp(559): error:
The enclosing parent function ("doBoruvka") for an extended __host__ __device__
lambda cannot have private or protected access within its class

[aprokop]

I was sure that having if (radius < lower_bound) inside traverse was fine for GPU with noop functor. But it's not. I may be hallucinating, but I rerun all commits in this PR and they all introduced 12% perf regression on A100.

So I did the final salute by just explicitly stripping the check completely out the functor by introducing tags, and dispatching based on the execution space.

I did not guard recomputing lower bounds for GPU, as it takes < 0.1%.