在模块化 python 代码库中使用 Dask LocalCluster()

Question

I am trying to use Dask Distributed's LocalCluster to run code in parallel using all the cores of a single machine.

Consider a sample python data pipeline, with the folder structure below.

sample_dask_program
├── main.py
├── parallel_process_1.py
├── parallel_process_2.py
├── process_1.py
├── process_2.py
└── process_3.py

main.py is the entry point, which executes while pipeline sequentially.

Eg:

def run_pipeline():
    stage_one_run_util()
    stage_two_run_util()

    ...

    stage_six_run_util()


if __name__ == '__main__':

    ...

    run_pipeline()

parallel_process_1.py and parallel_process_2.py are modules which create a Client() and use futures to achieve parallelism.

with Client() as client:
            # list to store futures after they are submitted
            futures = []

            for item in items:
                future = client.submit(
                    ...
                )
                futures.append(future)

            results = client.gather(futures)

process_1.py , process_2.py and process_3.py are modules which do simple computation that need not be run in parallel using all the CPU cores.

Traceback:

  File "/sm/src/calculation/parallel.py", line 140, in convert_qty_to_float
    results = client.gather(futures)
  File "/home/iouser/.local/lib/python3.7/site-packages/distributed/client.py", line 1894, in gather
    asynchronous=asynchronous,
  File "/home/iouser/.local/lib/python3.7/site-packages/distributed/client.py", line 778, in sync
    self.loop, func, *args, callback_timeout=callback_timeout, **kwargs
  File "/home/iouser/.local/lib/python3.7/site-packages/distributed/utils.py", line 348, in sync
    raise exc.with_traceback(tb)
  File "/home/iouser/.local/lib/python3.7/site-packages/distributed/utils.py", line 332, in f
    result[0] = yield future
  File "/home/iouser/.local/lib/python3.7/site-packages/tornado/gen.py", line 735, in run
    value = future.result()
concurrent.futures._base.CancelledError

This is the error thrown by the workers:

distributed.worker - ERROR - failed during get data with tcp://127.0.0.1:33901 -> tcp://127.0.0.1:38821
Traceback (most recent call last):
  File "/home/iouser/.local/lib/python3.7/site-packages/distributed/comm/tcp.py", line 248, in write
    future = stream.write(frame)
  File "/home/iouser/.local/lib/python3.7/site-packages/tornado/iostream.py", line 546, in write
    self._check_closed()
  File "/home/iouser/.local/lib/python3.7/site-packages/tornado/iostream.py", line 1035, in _check_closed
    raise StreamClosedError(real_error=self.error)
tornado.iostream.StreamClosedError: Stream is closed
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/iouser/.local/lib/python3.7/site-packages/distributed/worker.py", line 1248, in get_data
    compressed = await comm.write(msg, serializers=serializers)
  File "/home/iouser/.local/lib/python3.7/site-packages/distributed/comm/tcp.py", line 255, in write
    convert_stream_closed_error(self, e)
  File "/home/iouser/.local/lib/python3.7/site-packages/distributed/comm/tcp.py", line 121, in convert_stream_closed_error
    raise CommClosedError("in %s: %s: %s" % (obj, exc.__class__.__name__, exc))
distributed.comm.core.CommClosedError: in <closed TCP>: BrokenPipeError: [Errno 32] Broken pipe

I am not able to locally reproduce this error or find a minimum reproducible example, as the occurrence of this error is abrupt.

Is this the right way to use Dask LocalCluster in a modular python program?

EDIT

I have observed that these errors come up when the LocalCluster is created with a relatively high number of threads and processes. I am doing computations which uses NumPy and Pandas and this is not a good practice as described here .

At times, when the LocalCluster is created using 4 workers and 16 processes, no error gets thrown. When the LocalCluster is created using 8 workers and 40 processes, the error I described above gets thrown.

As far as I understand, dask randomly selects this combination (is this an issue with dask?), as I tested on the same AWS Batch instance (with 8 cores (16 vCPUs)).

The issue does not pop up when I forcefully create the cluster with only threads.

Eg:

cluster = LocalCluster(processes=False)
with Client(cluster) as client:
    client.submit(...)
    ...

But, creating the LocalCluster using only threads slows down the execution by 2-3 times.

So, is the solution to the problem, finding the right number of processes/threads suitable to the program?

Answer 1

It is more common to create a Dask Client once, and then run many workloads on it.

with Client() as client:
    stage_one(client)
    stage_two(client)

That being said, what you're doing should be fine. If you're able to reproduce the error with a minimal example, that would be useful (but no expectations).