Not working with multiple processes

[YutingZhang]

When calling MobulaOP in a subprocess, it gets stuck.

Environment: lastest mxnet nightly build and Python 3.6.5

An example code modified from dynamic_import_op.py to replicate this error.

from concurrent import futures

import sys
import mxnet as mx

def foo():
    import mobula
    # Import Custom Operator Dynamically
    mobula.op.load('./AdditionOP')
    AdditionOP = mobula.op.AdditionOP

    a = mx.nd.array([1, 2, 3])
    b = mx.nd.array([4, 5, 6])

    a.attach_grad()
    b.attach_grad()

    with mx.autograd.record():
        c = AdditionOP(a, b)

    dc = mx.nd.array([7, 8, 9])
    c.backward(dc)

    assert ((a + b).asnumpy() == c.asnumpy()).all()
    assert (a.grad.asnumpy() == dc.asnumpy()).all()
    assert (b.grad.asnumpy() == dc.asnumpy()).all()

    print('Okay :-)')
    print('a + b = c \n {} + {} = {}'.format(a.asnumpy(), b.asnumpy(), c.asnumpy()))

def main():
    ex = futures.ProcessPoolExecutor(1)
    r = ex.submit(foo)
    r.result()

if __name__ == "__main__":
    main()

[wkcn]

Thanks for your report!
I will check it.

[YutingZhang]

Thanks!

FYI, If you move import mxnet as mx into foo(), the bug can disappear. But this is generally not doable because mxnet is usually imported in the main process. It may related to how mxnet works with subprocesses.

[wkcn]

moving import mobula and mobula.op.load('./AdditionOP') outside foo() may work, since MobulaOP will register operator into MXNet when mobula.op.load('./AdditionOP') is called.
I will add a check to avoid duplicated register.

[YutingZhang]

I tried that, but it does not work. Example code:

from concurrent import futures

import sys
import mxnet as mx

import mobula
# Import Custom Operator Dynamically
mobula.op.load('./AdditionOP')

def foo():

    AdditionOP = mobula.op.AdditionOP

    a = mx.nd.array([1, 2, 3])
    b = mx.nd.array([4, 5, 6])

    a.attach_grad()
    b.attach_grad()

    with mx.autograd.record():
        c = AdditionOP(a, b)

    dc = mx.nd.array([7, 8, 9])
    c.backward(dc)

    assert ((a + b).asnumpy() == c.asnumpy()).all()
    assert (a.grad.asnumpy() == dc.asnumpy()).all()
    assert (b.grad.asnumpy() == dc.asnumpy()).all()

    print('Okay :-)')
    print('a + b = c \n {} + {} = {}'.format(a.asnumpy(), b.asnumpy(), c.asnumpy()))

def main():
    ex = futures.ProcessPoolExecutor(1)
    r = ex.submit(foo)
    r.result()

if __name__ == "__main__":
    main()

[wkcn]

@YutingZhang
Hi! I found the bug is not related to MobulaOP.
It seems that MXNet triggers the bug.

from concurrent import futures

import mxnet as mx
import sys
from mobula.testing import assert_almost_equal
sys.path.append('../../')  # Add MobulaOP Path

class AdditionOP(mx.operator.CustomOp):
    def __init__(self):
        super(AdditionOP, self).__init__()
    def forward(self, is_train, req, in_data, out_data, aux):
        out_data[0][:] = in_data[0] + in_data[1]
    def backward(self, req, out_grad, in_data, out_data, in_grad, aux):
        in_grad[0][:] = out_grad[0]
        in_grad[1][:] = out_grad[0]

@mx.operator.register("AdditionOP")
class AdditionOPProp(mx.operator.CustomOpProp):
    def __init__(self):
        super(AdditionOPProp, self).__init__()
    def list_arguments(self):
        return ['a', 'b']
    def list_outputs(self):
        return ['output']
    def infer_shape(self, in_shape):
        return in_shape, [in_shape[0]]
    def create_operator(self, ctx, shapes, dtypes):
        return AdditionOP()

def foo():
    a = mx.nd.array([1, 2, 3])
    b = mx.nd.array([4, 5, 6])

    a.attach_grad()
    b.attach_grad()

    print("REC")
    with mx.autograd.record():
        c = mx.nd.Custom(a, b, op_type='AdditionOP')

    dc = mx.nd.array([7, 8, 9])
    c.backward(dc)

    assert_almost_equal(a + b, c)
    assert_almost_equal(a.grad, dc)
    assert_almost_equal(b.grad, dc)

    print('Okay :-)')
    print('a + b = c \n {} + {} = {}'.format(a.asnumpy(), b.asnumpy(), c.asnumpy()))

def main():
    ex = futures.ProcessPoolExecutor(1)
    r = ex.submit(foo)
    r.result()

if __name__ == '__main__':
    main()

[YutingZhang]

So mx.nd.Custom is the actual problem ... MxNet just has lots of bugs when running in subprocess ...

[wkcn]

Yes.

[YutingZhang]

@wkcn Send you an email to your live.cn email :)

[wkcn]

Mail received. Thank you! : )

[wkcn]

Hi @YutingZhang , the two testcases you gave have been passed in the latest MXNet and MobulaOP : )

[YutingZhang]

@wkcn Thanks a lot! Did you work around the problem in MobulaOP? Or is it due to MxNet's update on CustomOP (you also contributed to this)?

[wkcn]

@YutingZhang It is due to MXNet’s update, and other contributors fixed it.

[wkcn]

Close it since the problem has been addressed. : )