Feature/probinso/fast schedule dependents

[robinson96]

Summary

Adds reusable ATS ready-queue helpers for schedulers that cache structurally
ready work and choose the largest priority bucket that meets a user defined readiness criteria.

Changes

• Add ats.ready_queue.ReadyWorkSet for priority-binned ready work.
• Add focused scheduler extension documentation for cached ready schedulers,
  including a copyable minimal scheduler example.
• Add unit test covering the documentation example.

comments

This is the first of ~6 upcoming branches enabling reduction in scheduling overheads.

[ryanq4]

It looks like we could use start_available for step() but right now step also returns true if there is still work to do. Can ready_count be used for this or do we still need to maintain a list of groups and check for CREATED (or something similar)?

[robinson96]

If I'm understanding your question, ready_count would exclude tests that are CREATED but blocked by dependencies, which I think would be different than what you're referring to.

[ryanq4]

I might be looking too far ahead but I was thinking about this loop and probably trying to prematurely optimize. I was wondering if we could use ready_count to determine if work still needed to be done. That is, since the dependencies are being counted then would 0 ready_count indicate all work finished?

[ryanq4]

This looks like good infrastructure for future work.
Looking through it some important points I noticed were that order_lookup, priority_lookup, and serial_lookup need to remain stable for the duration of the ats run but that sounds reasonable to me.