Feature Request: Assign replicas to GPUs

[kheyer]

Description

I have a service that runs on a single GPU. If I have multiple GPUs available, I would like to create one replica of this service for each GPU available.

Currently, I can only do this by explicitly creating the service multiple times in the docker-compose file and changing the device_ids section of the resources

  resources:
    reservations:
      devices:
      - driver: "nvidia"
        device_ids: ["0"]
        capabilities: [gpu]

Having to create 8 near-identical replicas of the same service in the config file is unwieldy.

I would like to specify the service once and set the replicas with

replicas: ${GPU_COUNT}

However this requires some way for each replica to know what GPU to use. My understanding is currently there is no way to do this, and there isn't a good way to go off a "replica index" for each container (see #9153).

Some method of mapping replicas of a service to different GPUs would be helpful.

[ndeloof]

In an ideal world the engine (or nvidia driver) would manage this as a pool of resources, just like you can declare a service to bind port with a range, and let the engine select available port in the range, so that scaling is not an issue.

About #9153, I would not be comfortable we rely on container number used to index replicas. While we try to make this somehow sequential there are many corner cases and no guarantee you would always get value within the [1..GPU_COUNT] interval