hierarchical scheduling and timebands alan burns university of york, uk
Post on 19-Dec-2015
221 views
TRANSCRIPT
![Page 1: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/1.jpg)
Hierarchical Schedulingand Timebands
Alan Burns
University of York, UK
![Page 2: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/2.jpg)
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
![Page 3: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/3.jpg)
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
![Page 4: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/4.jpg)
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
![Page 5: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/5.jpg)
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
![Page 6: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/6.jpg)
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:
![Page 7: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/7.jpg)
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
![Page 8: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/8.jpg)
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
![Page 9: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/9.jpg)
9
Three Time Bands
E
A
![Page 10: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/10.jpg)
10
Topics for discussion
Planning and scheduling
Agents and resources
Hierarchical planning/scheduling
![Page 11: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/11.jpg)
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
![Page 12: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/12.jpg)
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
![Page 13: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/13.jpg)
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’
![Page 14: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/14.jpg)
14
Pure periodic problems
Can be by plan or by policy
Hence debate about priority-based or time-triggered ‘scheduling’
![Page 15: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/15.jpg)
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?
![Page 16: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/16.jpg)
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
![Page 17: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/17.jpg)
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
![Page 18: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/18.jpg)
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?
![Page 19: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/19.jpg)
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?
![Page 20: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/20.jpg)
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
![Page 21: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/21.jpg)
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)
![Page 22: Hierarchical Scheduling and Timebands Alan Burns University of York, UK](https://reader035.vdocuments.mx/reader035/viewer/2022062714/56649d395503460f94a13bf3/html5/thumbnails/22.jpg)
22
Precision
E2
A2
E1
A1