Federated Learning: Sparsification

The communication efficiency of federated learning is improved by sparsifying the parameters uploaded by the clients, hence reducing the size of the upload.

Three approaches to sparsification are compared:

• Random - Randomly selecting k% parameters
• Top-k - Selecting the top k% parameters with the largest absolute differences before and after model training
• Threshold - Selecting parameters with absolute differences that are larger than a given threshold

Two datasets (data.py) are used for the experiments:

• CIFAR-10 - Imbalanced, non-iid partition of data between the clients
• FEMNIST - Data distributed between the clients based on the author

The basic convolutional model CNN500k (models.py):

• Model for CIFAR-10 - 471,338 parameters (1.89 MB)
• Model for FEMNIST - 369,432 parameters (1.48 MB)

All experiments were implemented in PyTorch using Flower's virtual client engine (https://github.com/adap/flower).

Experiments

72 experiments were run for all combinations of:

• Dataset = [femnist, cifar]
• Approach = [random, topk, threshold]
• Sparsify by:
  • Random & Top-k = [0.5, 0.3, 0.1, 0.05, 0.03, 0.01]
  • Threshold = [0.001, 0.003, 0.005, 0.007, 0.0085, 0.01]
• Keep first and last layer = [TRUE, FALSE]

Results from the experiments: data and figures.

Experimental set-up:

• Number of clients = 50
• Number of epochs = 1
• Learning rate = 0.1
• Optimiser = SGD
• Regularisation = 0
• Fraction of clients sampled each round:
  • FEMNIST = 0.25
  • CIFAR = 0.3
• Number of federated learning rounds:
  • FEMNIST = 30
  • CIFAR = 180
• Metric = Accuracy as measured on the test dataset

Baseline:

Federated Averaging (https://arxiv.org/abs/1602.05629) using the same set-up as above.

Simulation

Example command:
python simulation.py --dataset_name="cifar" --approach="topk" --sparsify_by=0.1 --num_rounds=180

Requires version flwr==1.6.0. Version flwr==1.8.0 has bugs, as per #1 .

Parameter	Description
--dataset_name	Can be femnist or cifar.
--femnist_location	Path to the location of the femnist data. Must be pre-downloaded.
--approach	Can be random topkor threshold.
--sparsify_by	Float between 0 and 1 indicating the fraction of parameters to select. For the threshold approach this corresponds to the threshold value.
--num_rounds	Number of federated learning rounds.
--keep_first_last	Boolean TRUE or FALSE. Indicates whether to force the selection of all parameters in the very first and last layers in the network.
--epochs	Number of epochs each client trains the local model for.
--learning_rate	Learning rate for the model training.
--regularisation	Regularisation/weight decay parameter for the optimiser.