[d0mine]

1000 threads can run in parallel. It doesn't prevent us to sum their results deterministically:

    results = ThreadPool(workers=1000).imap_unordered(calc, inputs)
    print(math.fsum(results))

Due to the magic of the fsum alg, the result is deterministic whatever order we get results in. https://docs.python.org/3/library/math.html#math.fsum

[cpgxiii]

That's not the operation being performed on GPUs that is the problem. The issue is that fundamentally GPUs allow for high performance operations using atomics, but this comes at the cost of nondeterministic results. You can get deterministic results but doing so comes with a significant performance costs.

[xiphias2]

Using atomics is easier than warp operations (using warp shuffle for example), but warp shuffle is quite fast.

I guess if determinism is so important implementations can be changed, it is just maybe not that high priority.

[WithinReason]

That summation is slow and would not be used in practice.

You could use just one thread on your 10000 thread GPU too and it would be deterministic, sure. Completely beside the point.