Numba workqueue threading error

Thanks for the great library!

I’m getting the following error: Terminating: Nested parallel kernel launch detected, the workqueue threading layer does not supported nested parallelism. Try the TBB threading layer.

The context is that I’m calling the umap library from flask and two clients are hitting a umap call at the same time. I’m guessing this is what is causing nested threading – flask is already using some kind of threading, and umap seems to be doing the same under the hood. I think this is related to numba threading, but I’m setting a random seed so as per the docs I don’t think this should be an issue.

Is there a proposed/preferred solution to this issue?

[celinesin]

And, in the end my problem was incorrect wsgi configuration… what a treasure hunt! It works now, thanks for all the support!

[stuartarchibald]

The most common cause of this issue is nesting parallel kernels, here’s an example:

from numba import njit, prange
import numpy as np

@njit(parallel=True)
def nested(x):
    for i in prange(len(x)):
        x[i] += 1


@njit(parallel=True)
def main():
    Z = np.zeros((5, 10))
    for i in prange(Z.shape[0]):
        nested(Z[i]) # A parallel region is calling another function with a parallel region!
    return Z

main()

The workqueue threading layer does not support nested kernels, whereas OpenMP and TBB do. The message you are seeing is coming from the workqueue threading layer as it’s detected that the code does something it doesn’t support and is “protecting” itself from inevitable corruption that would occur were it to attempt to run it.

My first question is… do you have something like the case above or do you have a parallel kernel being called from two python threads? I think the effect would manifest in the same error message as the workqueue backend isn’t threadsafe and it would “see” parallel region launches from two python threads as a kind of nest.