WIP: Multithreading

[ranjanan]

Error I'm stuck on just now: when I'm running multi-threaded, I get NaNs for my voltages

[ranjanan]

Steps to reproduce:

1. Check out Circuitscape with this branch.
2. Start Julia with multiple threads
3. From the REPL run, buzz = Ref{Any}() and foo = Vector{Any}()
4. Run cd(joinpath(pkgdir(Circuitscape), "test")
5. Run compute(test_problem("sgVerify1.ini")

[ViralBShah]

What exactly are you multi-threading here? I couldn't tell from the diff - so would help to have a quick explanation.

[ranjanan]

I am trying to spawn several tasks that independently solve linear systems and generate and write current maps

[ViralBShah]

@ranjanan Can you document what you tried here and experiments with RR?

[ranjanan]

Currently runs but with returns incorrect result on raster pairwise. PR for AMG submitted: JuliaLinearAlgebra/AlgebraicMultigrid.jl#68

[ViralBShah]

Say I have 4 cores, and hence 4 threads, but I spawn 100 solves. Won't those 100 solves be concurrently updating this data structure, unless you schedule the solves to ensure that each processor is only working on one solve at a time?

I think you need to implement locking in the AMG code, so that each thread that is solving has a lock on the AMG data structure until its job is done, you should get it to work correctly.

[vlandau]

Won't those 100 solves be concurrently updating this data structure, unless you schedule the solves to ensure that each processor is only working on one solve at a time?

I had a similar issue with Omniscape, which is why I had to expand the array along a third dimension, with one slice per thread -- not the most ideal solution memory-wise, but not sure what the lock/unlock overhead might be with so many solves (can be one solve per second or even faster).

[ViralBShah]

Seeing errors with SharedArrays makes me feel that we really should just get multi-threading done. I feel like we were really close the last time and need to correctly handle a couple of races in AMG.

[ranjanan]

Yes, you are right. Trying to find bandwidth, which is a little elusive.

[ranjanan]

Will try to make a push this month