Straggler Handling Interface

The Open Federated Learning (OpenFL) framework supports straggler handling interface for identifying stragglers or slow collaborators for a round and ending the round early as a result of it. The updates from these stragglers are not aggregated in the aggregator model.

The following are the straggler handling algorithms supported in OpenFL:

CutoffTimeBasedStragglerHandling
Identifies stragglers based on the cutoff time specified in the settings. Arguments to the function are:

  • Cutoff Time (straggler_cutoff_time), specifies the cutoff time by which the aggregator should end the round early.

  • Minimum Reporting (minimum_reporting), specifies the minimum number of collaborators needed to aggregate the model.

For example, in a federation of 5 collaborators, if straggler_cutoff_time (in seconds) is set to 20 and minimum_reporting is set to 2, atleast 2 collaborators (or more) would be included in the round, provided that the time limit of 20 seconds is not exceeded. In an event where minimum_reporting collaborators don’t make it within the straggler_cutoff_time, the straggler handling policy is disregarded.

PercentageBasedStragglerHandling
Identifies stragglers based on the percetage specified. Arguments to the function are:

  • Percentage of collaborators (percent_collaborators_needed), specifies a percentage of collaborators enough to end the round early.

  • Minimum Reporting (minimum_reporting), specifies the minimum number of collaborators needed to aggregate the model.

For example, in a federation of 5 collaborators, if percent_collaborators_needed is set to 0.8 and minimum_reporting is set to 1, the first 4 collaborators (80%) to report to aggregator are selected for that round.

Demonstration of adding the straggler handling interface

The example template, torch_cnn_mnist_straggler_check, uses the PercentageBasedStragglerHandling. To gain a better understanding of how experiments perform, you can modify the percent_collaborators_needed or minimum_reporting parameter in the template plan.yaml or even choose CutoffTimeBasedStragglerHandling function instead:

straggler_handling_policy :
    template : openfl.component.straggler_handling_functions.CutoffTimeBasedStragglerHandling
    settings :
        straggler_cutoff_time : 20
        minimum_reporting : 1