Add cache for exchange and DMI pairs.

MariusSchiffer commented 5 years ago

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[x] I have read the README
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Description

This PR creates a cache for DMI/Exchange indices, resulting in a speedup of roughly 3x. Tests and sanity checks imply that it is correct.

Missing: Caches also have to be rebuilt when editing the pairfields. CUDA implementation. Further verification for correctness, especially for non-OpenMP builds.

coveralls commented 5 years ago

Coverage remained the same at 84.983% when pulling 9b1e01f60855d29420edf7682b1e48f970c8189f on MariusSchiffer:feature-hamiltonian-cache into 8b396c8c5a747ce23c6f2122bfddeceaef48a131 on spirit-code:develop.

GPMueller commented 5 years ago

I just noticed one major problem with this implementation: the cached pairs potentially overlap, i.e. there will be multiple pairs with the same ispin and/or jspin.

This means that when data is assigned to the energy or energy gradient of a spin, a race condition appears and the data may become incorrect. I am quite surprised that the unit tests did not catch this...

The optimal fix for this would be to have one list per spin, so that one could parallelize over all spins, where each can access its own list of neighbours.

Another problem is the potential memory consumption, which would require there to be an off-switch for the case of large system sizes (> 100^3).

I have to give this some thought, as I am not sure how to combine these different aspects properly and without duplication of code.

GPMueller commented 3 years ago

As the bugs were never adressed, I will close this PR now

spirit-code / spirit