The Spectrum of Distributed Machine Learning Algorithm

Current machine learning problem typically is associated with the large scale of dataset and highly complexity of the model. This provides us lots of ascpects and options to design the algorithm so that the multi-core CPU/GPU in the distributed computation system can be fully exploited. We list the most relevant and important considerations here.

• From the aspect of data and modeling concurrency:

  1. Data Parallelism
  2. Model Parallelism
  3. Pipeline Parallelism

  The above three techniques are not exclusive to each other. For example, tensorflow allow users can utilize all three techniques at the same time. The Bluefog project focused on Data Parallelism. One reason, of course, is the base algorithm derived based on the assumption that the dataset is distributed over different nodes. But, more importantly, among these three techniques, data parallelism is the most popular approaches thanks to its excellent scalability and flexibility on almost any model.

• From the aspect of communication architecture:

  1. Parameter Server(PS) —- (Distributed but still centralized)

  • Sharded PS
  • Hierarchical PS

  2. Peer-to-Peer —– (Distributed but also decentralized)

  • Ring-AllReduce
  • Neighbor-Collective

  Apparently, Bluefog project belongs to the peer-to-peer model. Multiple nodes/machines will distributedly and no centralized node will gather all the informations.

• From the aspect of parameter consistency:

  In the distributed learning system, parameter consistency means the similarity between the parameter stored in the local machine. We list five typical algorithms from strongest consistency to weakest consistency.

  1. Model Replication
  2. Delayed Updating (like asynchronous algorithm.)
  3. Model Averaging
  4. Ensemble Learning

  Bluefog project focused on the asynchronous training through the diffusion/consensus algorithm, which is one kind of model averaging algorithms. The parameter learned in different nodes through the Bluefog algorithm are slightly different. But unlike ensemble learning, all nodes are highly similar.

• From the aspect of updating synchronization:

  1. Synchronous updating
  2. Stale-Synchronous updating
  3. Asynchronous updating

  Typically, the more “asynchronous” updating, the faster on the training. However, we will loss the parameter consistency.

• From the aspect of information fusion:

  1. Averaging over the gradients
  2. Averaging over the parameter/iterates
  3. Averaging over the dual variable

  Most strong consistency algorithm is averaging over the gradients. However, Bluefog project is averaging over the parameter directly. One advantage of averaging over the parameter is the resilient on the noise or error. Also, noticing these three methods are not exclusive.

• From the aspect of reducing communication cost:

  1. Temporal compression (Fine- vs Coarse-Grained Fusion)
  2. Spatial compression (Sparse/Sliced tensors)
  3. Btye compression (Quantization)
  4. Neighbor compression (Selecting less neighbors)

  We don’t have any implementation to support it yet. We do plan to support it in the future.