Design: Standalone supports GPU scheduling

Design draft:

Scenarios:

• client-mode, worker might create one or more executor processes, from different Spark applications.
• cluster-mode, worker might create driver process as well.
• local-cluster model, there could be multiple worker processes on the same node. This is an undocumented use of standalone mode, which is mainly for tests.
• Resource isolation is not considered here.

Because executor and driver processes on the same node will share the accelerator resources, worker must take the role that allocates resources. So we will add spark.worker.resource.[resourceName].discoveryScript conf for workers to discover resources. User need to match the resourceName in driver and executor requests. Besides CPU cores and memory, worker now also considers resources in creating new executors or drivers.

Example conf:

# static worker conf
spark.worker.resource.gpu.discoveryScript=/path/to/list-gpus.sh

# application conf
spark.driver.resource.gpu.amount=4
spark.executor.resource.gpu.amount=2
spark.task.resource.gpu.amount=1

In client mode, driver process is not launched by worker. So user can specify driver resource discovery script. In cluster mode, if user still specify driver resource discovery script, it is ignored with a warning.

Supporting resource isolation is tricky because Spark worker doesn't know how to isolate resources unless we hardcode some resource names like GPU support in YARN, which is less ideal. Support resource isolation of multiple resource types is even harder. In the first version, we will implement accelerator support without resource isolation.

Timeline:
1. Worker starts.
2. Worker loads `work.source.*` conf and runs discovery scripts to discover resources.
3. Worker reports to master cores, memory, and resources (new) and registers.
4. An application starts.
5. Master finds workers with sufficient available resources and let worker start executor or driver process.
6. Worker assigns executor / driver resources by passing the resource info from command-line.
7. Application ends.
8. Master requests worker to kill driver/executor process.
9. Master updates available resources.