02/03/2020, 3:15 PM
Hi, When using tags for dask resources, I observe a strange behavior while using mapping. Let's consider the following flow:
Copy code
map_fn = FunctionTask(lambda x: x + 1, tags=["dask-resource:GPU=1"])

with Flow('test') as flow:
    list = [1]
While running such a flow on a dask cluster with 1 scheduler, and 1 worker (with
--resources GPU=1
), it turns out that the flow is blocked because there are 2 tasks that "reserved" the gpu resources (see attached pics from the dask dashboard). It looks like the map itself reserved 1 GPU, and then the map_fn map on element from numbers cannot be processed because dask wait for the resource to be released. Is that the expected behavior of the mapping with dask resources?

Chris White

02/03/2020, 4:22 PM
Hi @Romain! this is actually expected but I agree it is not ideal; at the moment, Prefect submits a copy of your task to the executor to determine whether it is ready to be mapped and if so, to actually submit the mapped work out via a
, which results in 1 extra resource being used than you might otherwise expect. This is up for a refactor very very soon, in which case you will stop seeing this behavior!
👏 2