Choosing between Schedulers

Dask enables you to run your computations on a variety of schedulers that use different technologies, ranging from a single thread, to multiple processes, to a distributed cluster. This document helps explain the differences between the different choices so that users can make decisions that improve performance.

Briefly, the current options are as follows:

  • dask.threaded.get: Uses multiple threads in the same process. Good for numeric code that releases the GIL (NumPy, Pandas, SKLearn, Numba) because data is free to share. The default scheduler for dask.array, dask.dataframe and dask.delayed
  • dask.multiprocessing.get: Uses multiple processes. Good for Python bound code that needs multiple interpreters to accelerate. There are some costs to sharing data back and forth between processes. The default scheduler for dask.bag and sometimes useful with dask.dataframe.
  • dask.get: Uses the single main thread. Good for profiling and debugging because all code is run sequentially
  • distributed.Client.get: Uses multiple machines connected over sockets. Good for larger work but also a viable alternative to dask.multiprocessing on a single machine. Also sometimes used for its improved diagnostic tools.

Threads vs Processes

Threads are good because they can share data back and forth in the same memory space without transfer costs. Threads can pass large arrays between each other instantaneously. Unfortunately due to the GIL pure Python code (like JSON parsing) does not parallelize well under threads, and so when computing on pure Python objects, like strings or lists or our custom objects, we may prefer to use processes. Threads are good when using numeric data and when the computation is complex with many cross-dependencies.

Processes don’t have issues with the GIL, but data transfer between cores when using processes can be expensive. Data transfer isn’t an issue for embarrassingly parallel computations, which is fortunately the norm when dealing with pure Python computations.

Single Threaded Scheduler

Debugging, profiling, and general comprehension of code is hard when computing in parallel. Standard tools like pdb or cProfile fail to operate well when running under multiple threads or processes. To resolve this problem there is a dask scheduler, dask.get that doesn’t actually run in parallel, but instead steps through your graph in the main thread. It otherwise operates exactly like the threaded and multiprocessing schedulers, and so is a faithful proxy when tracking down difficult issues.

Distributed Scheduler on a Cluster

The distributed scheduler is more sophisticated than the single machine schedulers (threaded, multiprocessing, and synchronous all share a common codebase). This is good because it can be significantly smarter about how it runs computations. However this also introduces extra conceptual overhead and potential setup costs.

The primary reason to use the distributed scheduler is to use multiple machines on a distributed cluster. This allows computations to scale to significantly larger problems. This doesn’t come for free though, as you will need to setup the distributed scheduler on those machines.

Distributed Scheduler on a Single Machine

It is also reasonable to use the distributed scheduler on a single machine. This is often recommended over the multiprocessing scheduler for the following reasons:

  1. The multiprocessing scheduler brings intermediate values back to the main process before sending them out again for new tasks. For embarrassingly parallel workloads, such as are common in dask.bag, this is rarely a problem because repeated tasks are fused together and outputs are typically small, like counts, sums, or filenames to which we have written results. However for more complex workloads like a blocked matrix multiply this can be troublesome. The distributed scheduler is sophisticated enough to track which data is in which process and so can avoid costly interprocess communication.
  2. The distributed scheduler supports a set of rich real-time diagnostics which can help provide feedback and diagnose performance issues.
  3. The distributed scheduler supports a larger API, including asynchronous operations and computing in the background.

You can create a local “cluster” and use this scheduler by default by creating a dask.distributed.Client with no arguments.

from dask.distributed import Client
client = Client()

You may prefer to use the multiprocessing scheduler over the distributed scheduler if the following hold:

  1. Your computations don’t involve complex graphs that share data to multiple tasks
  2. You want to avoid depending on Tornado, the technology that backs the distributed scheduler

Diagnostics

One reason to do this is to get access to the pleasant web interface, which gives a real-time visualization of what’s computing on your cores.

Asynchronous Interface

The distributed scheduler also provides asynchronous computation, where you can submit a computation to run in the background, and only collect its results later

Data Locality

The distributed scheduler is sometimes more efficient than the multiprocessing scheduler, particularly when tasks have complex dependency structures and require non-trivial communication.

Because of how the standard library’s multiprocessing.Pool works, the multiprocessing scheduler always brings intermediate results back to the master process and then sends them out to a worker process afterwards if further work needs to be done. This back-and-forth communication can dominate costs and slow down overall performance. The distributed scheduler does not have this flaw, can reason well about data-in-place, and can move small pieces of data to larger ones.