1. Hierarchical Scheduling and Timebands Alan Burns University of York, UK

2. 2 Timebands  Complex systems exist at a wide range of time scales  A timeband framework has been developed to use ‘time’ to separate concerns in systems design and architectural descriptions

3. 3 Example – power grid  Wave effects,eg lightening surges, <ms  Switching, ms  Fault protection, 100ms  Stability, second  Economic load dispatching, 10s+  Thermodynamic changes, minute+  Load management, hour  Load forecasting, day

4. 4 Example – power grid  Maintenance scheduling, month  New Build, year  Expansion planning, decade  Decommissioning, centuries  At all levels from nanosecond to centuries, planning and scheduling are needed

5. 5 IBM’s Real-Time Pyramid (Hierarchies and Layers) 10 μs 100 μ s 1 ms 10 ms 100 ms 1 s 10 s signaling sensing actuation coordination tactics strategy Perception reaction cognition Custom Hardware Hard Real-Time and/or Safety-Critical Soft Real-Time Traditional Non-Real-Time

6. 6 Timeband Framework  Use ‘time’ to separate concerns  A system consist of a finite set of partially ordered timebands  A time bands is primarily defined by its granularity (eg. Hour or millisecond band)  Slower bands are static  Faster bands are instantaneous  When giving a lecture:

7. 7 Phenomena  Common notions at each band  Periodicity – cyclic, pace, …  Deadlines – response times, completion times, …  Agents and resources  Scheduling – planning, ordering  Temporal validity - staleness  Agreement – coordination, consensus, control, …  Affordances – learning, adapting, robustness, …  Self-symmetry  Hierarchical (cascade) control  Hierarchical scheduling

8. 8 Framework - II  Within a band there are activities that take time (units of band granularity) and events that are instantaneous (occur within the precision of the band)  Activities require resources/agents  Two events can be at the same time but have a precedence relation  For example, open fridge door -> light comes on  Events in one band may map to activities in a faster band

9. 9 Three Time Bands E A

10. 10 Topics for discussion  Planning and scheduling  Agents and resources  Hierarchical planning/scheduling

11. 11 Scheduling and Planning  Organise the order and time at which activities occur in order to meet timing constraints  Involves agents and resources  Is it useful to draw a distinction between scheduling and planning

12. 12 Planning  A focus on coordinating the use of agents and (many) different physical resources to meet timing requirements, ie produce a plan  Problem is well defined (arrivals of ‘work’ and ‘execution’ times)  At one level a constraints satisfaction problem  Different stake-holders and QoS issues makes it a multi-objective optimisation problem  Proof by construction  Robustness – problem not perfectly defined  Time to produce plan usually not an issue, but need for re-planning

13. 13 Scheduling  More an emphasis on multi-purpose agents  Deal with not fully specified problem (e.g. when ‘work’ arrives)  Produces a policy (not a plan)  Policy ‘quick’ to apply  Allows predictions to be made  Policy could be ‘to make a plan’

14. 14 Pure periodic problems  Can be by plan or by policy  Hence debate about priority-based or time-triggered ‘scheduling’

15. 15 Agents and Resources  At all bands, agents and resources must be managed to meet timing requirements  Agents are general purpose and are capable of undertaking many different activities – some level of autonomy  Examples: organisations, teams, people, processors  Resources are passive but are needed for agents to accomplish their activities  Examples: power, data, buffers  Is this a useful/meaningful distinction?

16. 16 Hierarchical Scheduling  Timebands define levels of temporality  It is clear that in a complex system there are hierarchies of schedules:  plans within plans  policies within policies  plans within policies within plan  A resource at one band could be implemented by an agent at a lower band, and visa versa

17. 17 Hierarchical planning/scheduling  If the action at lower level can be taken to be instantaneous, and plan/policy at higher level is constant -  then composing plans within plans within policies with plans etc should work fine  And deal with exceptions  But if clear separation is not possible then efficiencies and/or failures are likely

18. 18 Multi-band agents  Agents may work at more than one band  Need scheduling at multiple bands  They could be subject to planning at one band but be creators of plans at another  Does this help produce more resilient systems?

19. 19 Conclusions  Complete systems exhibit behaviour at many different time scales  Use time to separate concerns  Agents and resources?  is there a difference?  Policies and Plans?  is there a difference?  Are there things to say about hierarchical composition?

20. 20 Phenomena - I  Cycles of behaviour  Each cycle an order of magnitude slower than the one below  Simon’s view of architectures  Newell’s cognitive hierarchy  Biological band, neuron activity, ms level  Cognitive band, operations, second level  Rational band, task, 10 minutes level  Social bands, cooperative task, week level

21. 21 Framework - III  Two events can be simultaneous but have a precedence relation  For example, open fridge door -> light comes on  Or be just simultaneous  Talk starts at 11.15  Simultaneous events in one band must occur with the precision of that band (as measured in a faster band)

22. 22 Precision E2 A2 E1 A1