Launch Tasks from Tasks

Sometimes it is convenient to launch tasks from other tasks. For example you may not know what computations to run until you have the results of some initial computations.

Motivating example

We want to download one piece of data and turn it into a list. Then we want to submit one task for every element of that list. We don’t know how long the list will be until we have the data.

So we send off our original download_and_convert_to_list function, which downloads the data and converts it to a list on one of our worker machines:

future = e.submit(download_and_convert_to_list, uri)

But now we need to submit new tasks for individual parts of this data. We have three options.

  1. Gather the data back to the local process and then submit new jobs from the local process
  2. Gather only enough information about the data back to the local process and submit jobs from the local process
  3. Submit a task to the cluster that will submit other tasks directly from that worker

Gather the data locally

If the data is not large then we can bring it back to the client to perform the necessary logic on our local machine:

>>> data = future.result()                  # gather data to local process
>>> data                                    # data is a list
[...]

>>> futures = e.map(process_element, data)  # submit new tasks on data
>>> analysis = e.submit(aggregate, futures) # submit final aggregation task

This is straightforward and, if data is small then it is probably the simplest, and therefore correct choice. However, if data is large then we have to choose another option.

Submit tasks from client

We can run small functions on our remote data to determine enough to submit the right kinds of tasks. In the following example we compute the len function on data remotely and then break up data into its various elements.

>>> n = e.submit(len, data)                 # compute number of elements
>>> n = n.result()                          # gather n (small) locally

>>> from operator import getitem
>>> elements = [e.submit(getitem, data, i) for i in range(n)]  # split data

>>> futures = e.map(process_element, elements)
>>> analysis = e.submit(aggregate, futures)

We compute the length remotely, gather back this very small result, and then use it to submit more tasks to break up the data and process on the cluster. This is more complex because we had to go back and forth a couple of times between the cluster and the local process, but the data moved was very small, and so this only added a few milliseconds to our total processing time.

Submit tasks from worker

Note: this interface is new and experimental. It may be changed without warning in future versions.

We can submit tasks from other tasks. This allows us to make decisions while on worker nodes.

To submit new tasks from a worker that worker must first create a new client object that connects to the scheduler. There is a convenience function to do this for you so that you don’t have to pass around connection information. However you must use this function local_client as a context manager to ensure proper cleanup on the worker.

from distributed import local_client

def process_all(data):
    with local_client() as e:
        elements = e.scatter(data)
        futures = e.map(process_element, elements)
        analysis = e.submit(aggregate, futures)
        result = analysis.result()
    return result

 analysis = e.submit(process_all, data)  # spawns many tasks

This approach is somewhat complex but very powerful. It allows you to spawn tasks that themselves act as potentially long-running clients, managing their own independent workloads.

Extended Example

This example computing the Fibonacci numbers creates tasks that submit tasks that submit tasks that submit other tasks, etc..

In [1]: from distributed import Client, local_client

In [2]: client = Client()

In [3]: def fib(n):
   ...:     if n < 2:
   ...:         return n
   ...:     else:
   ...:         with local_client() as c
   ...:             a = c.submit(fib, n - 1)
   ...:             b = c.submit(fib, n - 2)
   ...:             a, b = c.gather([a, b])
   ...:             return a + b
   ...:

In [4]: future = e.submit(fib, 100)

In [5]: future
Out[5]: <Future: status: finished, type: int, key: fib-7890e9f06d5f4e0a8fc7ec5c77590ace>

In [6]: future.result()
Out[6]: 354224848179261915075

This example is a bit extreme and spends most of its time establishing client connections from the worker rather than doing actual work, but does demonstrate that even pathological cases function robustly.

Technical details

Tasks that invoke local_client are conservatively assumed to be long running. They can take a long time blocking, waiting for other tasks to finish, gathering results, etc.. In order to avoid having them take up processing slots the following actions occur whenever a task invokes local_client.

  1. The thread on the worker running this function secedes from the thread pool and goes off on its own. This allows the thread pool to populate that slot with a new thread and continue processing additional tasks without counting this long running task against its normal quota.
  2. The Worker sends a message back to the scheduler temporarily increasing its allowed number of tasks by one. This likewise lets the scheduler allocate more tasks to this worker, not counting this long running task against it.

Because of this behavior you can happily launch long running control tasks that manage worker-side clients happily, without fear of deadlocking the cluster.

Establishing a connection to the scheduler takes on the order of 10-20 ms and so it is wise for computations that use this feature to be at least a few times longer in duration than this.