Coupling to NEST

[thorstenhater]

This is one stepping stone towards a CoSim workflow as described by HBP:SGA3 T5.5.

Minimalist Steering

• Receive KILL command
  • hard kill: UNIX signal
  • soft kill: command via, will stop after current call to simulation::run finishes
• Send heartbeat
  • 1st implementation via stdio
  • can leverage #1870
• Output minimal delay
  • via stdio
  • maybe JSON

Coupling

• Open a connection to another spiking simulator (or its proxies)
  • MPI_Comm_connect (https://www.mpich.org/static/docs/v3.2/www3/MPI_Comm_connect.html)
  • Metadata for connect must be transferred via stdio or similar measures
• Receive remote spikes in our native format over said intercommunicator
  • likely Intercomm Allgather works, but these are weird, so do some reading
  • one or two rounds of exchange? So, Arbor -> Arbor and then Arbor -> NEST, or do all in one
• Allow 'remote' connections during setup.
  • Will need an extra tag and/or method on recipe

Notes

• There's prior work by @apeyser in https://github.com/apeyser/arbor/tree/feature/nestio
  • Ideas look sane, but not mergeable anymore
• As said Intercomm collectives are weird, but here they do what we want?!
  • After the AllgatherV both Arbor and the coupled application have all slices of spikes
  • However the semantics imply that we need two rounds anyhow, so maybe AllgatherV(intra) and Broadcast(inter) is better?

[w-klijn]

Great write down. No specific remarks from my side. Mainly commenting so I get registered to the updates on this topic

[thorstenhater]

@w-klijn Any experience and/or suggestion on how to provide identifiers of the remote endpoints across the airgap? Since we do not know what lives on the other side assuming a simple integer is not enough (Arbor uses (cell_gid, offset_on_cell)). For the time being I'd go with a free form (ie string) field here, but this disables some optimisations on filtering spikes.

Also, related: Do we foresee N:M coupling, so for example Arbor, NEST, TVB in a triangle? Then we'd have to offer a way to dynamically attach more partners.