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Discussion: Scheduling across runner and SDKHarness in Portability framework


Hi,

tl;dr Dead Lock in task execution caused by limited task parallelism on SDKHarness.

Setup:
  • Job type: Beam Portable Python Batch Job on Flink standalone cluster.
  • Only a single job is scheduled on the cluster.
  • Everything is running on a single machine with single Flink task manager.
  • Flink Task Manager Slots is 1.
  • Flink Parallelism is 1.
  • Python SDKHarness has 1 thread.
Example pipeline:
Read -> MapA -> GroupBy -> MapB -> WriteToSink

Issue:
With multi stage job, Flink schedule different dependent sub tasks concurrently on Flink worker as long as it can get slots. Each map tasks are then executed on SDKHarness.
Its possible that MapB gets to SDKHarness before MapA and hence gets into the execution queue before MapA. Because we only have 1 execution thread on SDKHarness, MapA will never get a chance to execute as MapB will never release the execution thread. MapB will wait for input from MapA. This gets us to a dead lock in a simple pipeline.

Mitigation:
Set worker_count in pipeline options more than the expected sub tasks in pipeline.

Proposal:
  1. We can get the maximum concurrency from the runner and make sure that we have more threads than max concurrency. This approach assumes that Beam has insight into runner execution plan and can make decision based on it.
  2. We dynamically create thread and cache them with a high upper bound in SDKHarness. We can warn if we are hitting the upper bound of threads. This approach assumes that runner does a good job of scheduling and will distribute tasks more or less evenly.
We expect good scheduling from runners so I prefer approach 2. It is simpler to implement and the implementation is not runner specific. This approach better utilize resource as it creates only as many threads as needed instead of the peak thread requirement.
And last but not the least, it gives runner control over managing truly active tasks.

Please let me know if I am missing something and your thoughts on the approach.

Thanks,
Ankur