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Distributed Execution Backends

Backend config

The same Ludwig config / Python code that runs on your local machine can be executed remotely in a distributed manner with zero code changes. This distributed execution include preprocessing, training, and batch prediction.

In most cases, Ludwig will be able to automatically detect if you're running in an environment that supports distributed execution, but you can also make this explicit on the command line with the --backend arg or by providing a backend section to the Ludwig config YAML:

backend:
  type: local
  cache_format: parquet
  cache_dir: s3://my_bucket/cache

Parameters:

  • type: How the job will be distributed, one of local, ray, horovod.
  • cache_format: Representation of the preprocessed data in the cache, one of hdf5, parquet, tfrecord.
  • cache_dir: Where the preprocessed data will be written on disk, defaults to the location of the input dataset.

Horovod

You can distribute the training and prediction of your models using Horovod, which allows to train on a single machine with multiple GPUs as well as on multiple machines with multiple GPUs.

In order to use distributed training you have to install Horovod as detailed in Horovod's installation instructions (which include installing OpenMPI or other MPI implementations or Gloo) and then install the two packages:

pip install horovod mpi4py

Horovod works by, in practice, increasing the batch size and distributing a part of each batch to a different node and collecting the gradients from all the nodes in a smart and scalable way. It also adjusts the learning rate to counter balance the increase in the batch size. The advantage is that training speed scales almost linearly with the number of nodes.

experiment, train and predict commands accept a --backend=horovod argument that instructs the model building, training and prediction phases to be conducted using Horovod in a distributed way. A horovodrun command specifying which machines and / or GPUs to use, together with a few more parameters, must be provided before the call to Ludwig's command. For instance, in order to train a Ludwig model on a local machine with four GPUs one you can run:

horovodrun -np 4 \
    ludwig train ...other Ludwig parameters...

While for training on four remote machines with four GPUs each you can run:

horovodrun -np 16 \
    -H server1:4,server2:4,server3:4,server4:4 \
    ludwig train ...other Ludwig parameters...

The same applies to experiment, predict and test.

More details on Horovod installation and run parameters can be found in Horovod's documentation.

Ray

Ray is a framework for distributed computing that makes it easy to scale up code that runs on your local machine to execute in parallel across a cluster.

Ludwig has native integration with Ray for both hyperparameter search and distributed training.

Running with Ray has several advantages over local execution:

  • Ray enables you to provision a cluster of machines in a single command through its cluster launcher.
  • Horovod on Ray allows you to do distributed training without needing to configure MPI in your environment.
  • Dask on Ray allows you to process large datasets that don't fit in memory on a single machine.
  • Ray Tune allows you to easily run distributed hyperparameter search across many machines in parallel.
  • Ray provides easy access to high performance instances like high memory or GPU machines in the cloud.

All of this comes for free without changing a single line of code in Ludwig. When Ludwig detects that you're running within a Ray cluster, the Ray backend will be enabled automatically. You can also enable the Ray backend explicitly either through the command line:

ludwig train ... --backend ray

Or in the Ludwig config:

backend:
  type: ray
  data_format: parquet
  engine:
    type: dask

Running Ludwig with Ray

To use the Ray with Ludwig, you will need to have a running Ray cluster. The simplest way to start a Ray cluster is to use the Ray cluster launcher, which can be installed locally with pip:

pip install ray

Starting a Ray cluster requires that you have access to a node provider like AWS EC2 or Kubernetes.

Here's an example of a partial Ray cluster configuration YAML file you can use to create your Ludwig Ray cluster:

cluster_name: ludwig-ray-gpu-nightly

min_workers: 4
max_workers: 4

docker:
    image: "ludwigai/ludwig-ray-gpu:nightly"
    container_name: "ray_container"

head_node:
    InstanceType: c5.2xlarge
    ImageId: latest_dlami

worker_nodes:
    InstanceType: g4dn.xlarge
    ImageId: latest_dlami

This configuration runs on AWS EC2 instances, with a CPU head node and 4 GPU (Nvidia T4) worker nodes. Every worker runs within a Docker image that provides Ludwig and its dependencies, including Ray, Dask, Horovod, etc. You can use one of these pre-built Docker images as the parent image for your cluster. Ludwig provides both CPU and GPU images ready for use with Ray.

Once your Ray cluster is configured, you can start the cluster and submit your existing ludwig commands or Python files to Ray for distributed execution:

ray up cluster.yaml
ray submit cluster.yaml ludwig train --config config.yaml --dataset s3://mybucket/dataset.parquet