View
217
Download
4
Tags:
Embed Size (px)
Citation preview
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