charm4py.Reducer contains the reducer functions that have been registered with the runtime. Reducer functions are used in Reductions, to aggregate data across the members of a chare collection (see Chare).


Reducer has the following built-in attributes (reducers) for use in reductions:

  • max: max function. When contributions are vectors (lists or arrays) of numbers, the reduction result will be the pairwise or “parallel” maxima of the vectors.
  • min: min function. Pairwise minima in the case of vector contributions.
  • sum: sum function. Pairwise sum in the case of vector contributions.
  • product: product function. Pairwise product in the case of vector contributions.
  • nop: This is used for empty reductions which do not contribute any data. Empty reductions are useful to know when all the chares in a collection have reached a certain point (e.g. synchronization purposes). Passing None as reducer in contribute calls has the same effect.
  • gather: Adds contributions to a Python list, and sorts the list based on the index of the contributors in their collection.

Registering custom reducers

To register a custom reducer function:

Reducer.addReducer(func, pre=None, post=None)

where func is a Python function with one parameter (list of contributions), and must return the result of reducing the given contributions. pre is optional and is a function intended to pre-process data passed in Chare.contribute() calls. It must take two parameters (data, contributor), where data is the data passed in a contribute call and contributor is the chare object. post is optional and is a function intended to post-process the data after the whole reduction operation has completed. It takes only parameter which is the reduced data.

To refer to a custom reducer:, where name is the name of the function that was passed to addReducer.