bonded GPU kernel fusion
The launch overhead of the bonded kernels often becomes so significant that it outweighs any benefit of GPU offload. This could be mitigated with a few optimizations: most importantly kernel fusion.Multiple approaches are possible:
- a simple decomposition of different types of bonded interactions over different blocks
- a more locality aware-decomposition would however be beneficial: interactions that share the coordinates computed in the same block.
Additionally, update groups could also be implemented in the decomposition (e.g. through block sorting which should not be difficult if coordinate ranges are already the unit of work) to prioritize work on the critical path for staggered update as well as for DD runs.
#4 Updated by Szilárd Páll over 1 year ago
- kernels are severely memory/instruction limited mostly due to scattered coordinate loads and to a lesser extent non-vecorizable parameter loads
- due to alignment parameter loads can't always be vectorized (and even when they can, for adh on a 1070 for angles it did not help)
- the best approach would be efficiently loading coordinates but if time doesn't allow
- at least simple fusion within or across blocks should be implemented -- both would greatly improve cache reuse;
- fusion within blocks may be more efficient if there is other work to overlap with but how to map different interaction types to warps to avoid leaving lots of threads idle is not clear.
#5 Updated by Magnus Lundborg about 1 year ago
- Assignee set to Magnus Lundborg
I'm assigning this to myself for now. Hopefully I can do this, but will probably need some advice along the way.
"... there are two strategies "horizontal" and "vertical" fusion.
1) Horizontal, across GPU blocks: we'd need N_total = sum(N_type) threads launched, compute an index range for all types and map the GPU threadIndex to the individual bonded index ranges. This is relatively straightforward, but may not be optimal assuming there is other work to overlap with
2) Vertical: within GPU blocks: launch N_total = max(N_type) threads and call the different interaction types sequentially within the kernel; ideally we'd want to map such that warps/blocks reuse atom data at least from cache, but this would require scattering the interaction types across blocks, then compacting as much as possible to avoid empty warps. If we could do a simple but smart sorting into blocks, this may be more efficient overall (especially if it can be nearly as efficient a hypothetical fine-grained update-group based decomposition)."
I will go for alternative 1 for now. I realise that option 2 would be better, but I think it's currently good just to avoid the multiple kernel launches. Do I understand it correctly that for option 1 updateInteractionListsAndDeviceBuffers() could remain as it is?
#6 Updated by Szilárd Páll about 1 year ago
- Status changed from New to In Progress
I've just done some quick checks and for the GluCL (ion_channel_ benchmark, the virial + energy kernel got ~1.4x slower (compared to the sum of the wall-time of all equivalent kernels in the r2019). I've also checked narrower blocks and it seems that the atomic accumulation seems to be the culprit creating even more contention on the L2 cache (additionally to the low efficiency loads). Making blocks narrower seems to improve performance by nearly 2x (even 64-wide improves further, but I realize that without direct global reduction that won't work for the shift vectors).