Real-Time Computing
For Human Computer Interfacing
October 23, 1997
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I. What's Real-Time? Some Definitions
Card Oriented Batch | 100-10000s |
Keyboard Oriented Batch | 1-100s |
Interactive Computing | 1-10s |
Online Inquiry and Transactions | 1-10s |
Message Switching | 0.1-10s |
Data Acquisition and Control | 0.01-10s |
Ib. What's Real-Time? Continued
What do the Computer Companies Say?
- SGI: Media Playback is Synchronizable
to 1 Audio Sample
- Windows: Graphics Display Looks Smooth,
Audio Doesn't Click too Much
Prerecorded Game Sounds Play Quickly
- Apple, NeXT, Others: Still different definitions.
- Computer Company Definitions Typically Say Little
About Latency Guarantees!
to 1 Audio Sample
Audio Doesn't Click too Much
Prerecorded Game Sounds Play Quickly
About Latency Guarantees!
Ic. What's Real-Time? Continued
What do the Controls/Robotics People Say?
- Systems Function Under Time Constraints
- Imposed by External Processes
- Scheduling and Resource Allocation Guarantees
- Synchronous, or Guaranteed Timings (Stability)
II. Controls Systems vs. Media Delivery
Media Delivery:
- Make it look/sound smooth, degrade gracefully
- No clicks or pops in audio, no jerks in video.
Control Systems:
- Close a feedback loop with sensors and motors.
- Guarantee STABILITY.
HCI is Both:
We're really interested in combining two areas. We want to get user input,
process it in a timely fashion, and display something smoothly and convincingly
(with small or controllable delay) in response. The human and computer make
a complete feedback system. Many modern HCI applications are active
feedback control systems, and thus stability can be a consideration.
- Make it look/sound smooth, degrade gracefully
- No clicks or pops in audio, no jerks in video.
Control Systems:
- Close a feedback loop with sensors and motors.
- Guarantee STABILITY.
HCI is Both:
We're really interested in combining two areas. We want to get user input,
process it in a timely fashion, and display something smoothly and convincingly
(with small or controllable delay) in response. The human and computer make
a complete feedback system. Many modern HCI applications are active
feedback control systems, and thus stability can be a consideration.
III. Control Systems
- A Classic Feedback Control System
- Feedback allows system to control itself
- Human sets desired state, but is not part of feedback
- Must worry about stability of feedback systems
- Feedback allows system to control itself
- What are some systems where human is part of feedback?
IIIb. Control Systems
- Humans in the Feedback Loop:
- Cars and things you steer
- Musical Instruments (some, but not all)
- Theremin
- Brass
- Bowed
IIIc. Control Systems
- Car:
- Actuator: Steering system
- Mechanical System: Car, Wheels, Tires
- Human:
- Sensor: Visual Sensors (maybe more!)
- Controller: Brain and Arms
A More General Picture of a Feedback Control System:
IIId. Some Feedback Control Systems with Humans
- Broom Balancing
- Actuator: Hand
- System: Physics of Broomstick
- Sensor: Visual/Tactile/Kinesthetic
- Controller: Human
- Actuator: Hand/Joystick
- System: Computer running game software
- Sensor: Visual (auditory) System(s)
- Controller: Human
- Actuator: Hands
- System: Theremin electronics
- Sensor: Auditory
- Controller: Human
Video Game
Theremin
IIIe. Diagram of Control System with Human
- Problems:
- Latency
- Overcompensation, stability (gain)
- Feedback means possibility for instability
- System Delay (VR Sickness)
- Sampling and measurement errors
- Human overcompensation
- Variable Reaction time (attention, ...)
- Resolution
- Training
IV. Synchronous vs. Asynchronous Models
Polled vs. Interrupt Operation
Polled vs. Interrupt Operation
- Synchronous Real-Time
- Based on time-ordered external events.
Processor responds instantaneously to events.
(or time between events >> response time) - Asynchronous Real-Time
- Events at times indexed on the real numbers.
System responds within specified bounds.
V. Data and Processing Models
- Queues and Scheduling:
- FIFOs (fixed priority)
- Priority-based
- Interrupts:
- Single Priority
- Multi-Level
- Nestable
- Hardware vs. Software
Vb. Data and Processing Models Continued
- Process/Device Communication:
- Shared Memory
- Messages
- DMA
- Multi-Bus/Ring/Star/Common Bus
- Input/Output Buffers
- Ping Pong
- Single with Hi/Low Water
VI. Complexity!!!
Online: Scheduling, Interrupt Handling, Load Monitoring, ..Development: Debugging, Optimization, Profiling, ..
=> Need for an Operating System
VxWorks | . |
OS-9 | . |
VRTX | . |
LynxOS | . |
Chimera: | MC680x0 |
RT Mach | 80960, 80486, MIPS R3000 |
DSP/Media Processor Real-Time OS:
IBM MWave/OS | MWAVE DSP |
AT&T VCOS | DSP3210 |
SPOX | TICX0, Motorola 56x, ADI 21x |
Next Lecture:
Some Web References:
Educational Research Groups and Projects:
U. Michigan Real-Time Computing Lab
Carnegie Melon Real-Time Groups
Commercial:
Resource List Compiled by E. Douglas Jensen
Some Non-Web References Made from Dead Trees:
"Real-time programming : neglected topics"
Caxton C. Foster.
Reading, Mass. : Addison-Wesley Pub. Co., c1981.
"Real-time software for control : program examples in C"
David M. Auslander, Cheng H. Tham.
Englewood Cliffs, N.J. : Prentice Hall, c1990.
"Real-time systems, Specification, Verification, and Analysis,"
Mathai Joseph, ed.
Englewood Cliffs, N.J. : Prentice Hall, 1996
"Introduction to Real-time Software Design,"
S.T. Allworth and R.N. Zobel
New York, Springer Verlag, 1989
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