Multi-threading on a 2 CPU system

[fft]

The way you have your loops structured might be problematic.

Multithreading for nested loops

Indeed, lines like RM[i1,i1] += -GEself[1,1]/8π indicate that your code have data races. We can’t expect a deterministic behavior. FYI, there are a couple of examples in FLoops documentation for writing reduction of mutable objects without data races. See: In-place mutation and Initialization with @reduce(acc = op(init, x)) syntax. There’s also private variables support.

@threads for ..
    @threads for..

Nested @threads loops is probably a little hard to reason about what happens. If you restructured your code to have one top level parallel loop you would likely get better performance and more intuitive understanding of the parallel execution.