Do mapped tasks only behave in a parallel fashion on dask I Prefect Community #ask-community

Do mapped tasks only behave in a parallel fashion ...

Josh Greenhalgh

01/27/2021, 5:24 PM

Do mapped tasks only behave in a parallel fashion on dask? I am running on k8s using job per flow and all my mapped tasks log in order sequentially - also theres a huge gap between two mapped tasks I was under the impression that if you have;

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p1_res = extract_raw_from_api.map(
        asset_location=asset_locations,
        extract_date=unmapped(extract_date),
        extract_period_days=unmapped(extract_period_days),
    )
    p2_res = process_2.map(p1_res)
    p3_res = process_3.map(p2_res)

Then as soon as task 1 from p1_res is done then the corresponding task should start in process_2? As it stands no process_2 tasks start until many of the extract_raw_from_api tasks are already complete...

josh

01/27/2021, 5:31 PM

Hi @Josh Greenhalgh, yes parallel execution in prefect is accomplished by using a dask executor

Josh Greenhalgh

01/27/2021, 5:31 PM

Ok and theres no way to turn the mapped tasks into individual k8s jobs?

josh

01/27/2021, 5:32 PM

Not at this time but it is something we have heard requests about and are actively exploring!

Josh Greenhalgh

01/27/2021, 5:33 PM

So on k8s (without dask) theres no parallelism at all? That makes me sad 😞

Josh Greenhalgh

01/27/2021, 5:33 PM

Thanks @josh

josh

01/27/2021, 5:33 PM

Technically if you wanted to you could map over the k8s CreateNamespacedJob task where each task would execute an independent kubernetes job but that may be out of scope for what you want

Josh Greenhalgh

01/27/2021, 5:34 PM

Thats actually my next thought...

Josh Greenhalgh

01/27/2021, 5:34 PM

It would be really great to have an executor that spat out k8s jobs for every task!

Jim Crist-Harif

01/27/2021, 5:35 PM

Note that you don't need to use a distributed executor (

DaskExecutor

) to get parallelization, but you do need to use something other than the default

LocalExecutor

. If your tasks are all io bound (for example, starting and monitoring external k8s jobs), you might find the

LocalDaskExecutor

sufficient. This uses a local thread pool to run tasks in parallel.

Jim Crist-Harif

01/27/2021, 5:35 PM

See https://docs.prefect.io/orchestration/flow_config/executors.html#choosing-an-executor for more info.

Josh Greenhalgh

01/27/2021, 5:36 PM

Ohhh so that would start a dask cluster within the pod and then use that?

Josh Greenhalgh

01/27/2021, 5:37 PM

Well not a cluster I suppose but a dask scheduler...

Jim Crist-Harif

01/27/2021, 5:37 PM

Yeah

Josh Greenhalgh

01/27/2021, 5:38 PM

Ok thanks! Will give that a go!

Josh Greenhalgh

01/27/2021, 5:43 PM

Only change I made was adding the executor;

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with Flow(name=name, storage=storage, run_config=run_config, schedule=schedule, executor = DaskExecutor()) as flow:

And now my mapped tasks don't even start I get a pickling error about max recursion depth 😞

Jim Crist-Harif

01/27/2021, 6:09 PM

Hmmm, sounds like an issue with your tasks. Can you provide the traceback, or a reproducible example?

Jim Crist-Harif

01/27/2021, 6:10 PM

(also note that f you swap out

executor=LocalDaskExecutor()

you won't get runtime pickling, since all your tasks run in the same process. This shouldn't be required though, the error you got above isn't expected).

Josh Greenhalgh

01/27/2021, 6:13 PM

Yeah I am having trouble with everything! I think fist thing to solve is my puny nodes and then maybe this will go away...

Josh Greenhalgh

01/27/2021, 6:14 PM

Thanks @Jim Crist-Harif and all the other really responsive prefect folks!

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