Hierarchical Scheduling and Timebands Alan Burns University of York, UK

Hierarchical Schedulingand Timebands

Alan Burns

University of York, UK

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

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

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

IBM’s Real-Time Pyramid (Hierarchies and Layers)

10 μs

100 μs

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/orSafety-Critical

Soft Real-Time

Traditional Non-Real-Time

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

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

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

Three Time Bands

E

A

10

Topics for discussion

Planning and scheduling

Agents and resources

Hierarchical planning/scheduling

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

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

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

Pure periodic problems

Can be by plan or by policy

Hence debate about priority-based or time-triggered ‘scheduling’

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

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

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

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

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

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

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

Precision

E2

A2

E1

A1