An Overview of Distributed RealTime Systems Research

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An Overview of Distributed Real-Time Systems
Research

• By Brian Demers
• March 24, 2003
• CS 535, Spring 2003

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Project Overview

• Examine efforts to combine real-time and
  distributed computing ideas
• Summarize trends, focal points
• Tried not to filter ideas
• Examine one or more sample implementations (if
  possible)
• Work in progress!

3
Presentation Overview

• Definitions of key terms, issues
• Discussion of key research areas, developments
• Networks
• Scheduling
• To-do list
• Conclusions, questions, suggestions

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Definitions

• Hard vs. soft real-time
• Distributed systems
• Did not consider traditional parallel computers
• General focus soft RT systems

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Networking
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Networking

• Inherently non-deterministic
• Research focus is on LAN configurations
• Main approaches
• Improve determinism of existing protocols
• Use probabilistic approach

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Ethernet

• Pros
• Cheap
• Widely used
• Cons
• Not designed for real-time use
• Packet arrival times can vary widely

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Ethernet Improvements

• Halmsted University
• Hardware solution Propose a network connected
  exclusively by switches (eliminating collisions)
• Switches perform data buffering, manage real-time
  streams

Switch
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Ethernet Improvements (cont.)

• Traffic smoothing (Kweon, Shin, and Workman)
• Probabilistically limit traffic into network
• Virtual token ring (Chiueh and Venkatramani)
• Switch to token-based scheme when RT traffic
  detected

Higher Network Layers
Regulator
Data-link Layer
Traffic smoothing approach
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Token-Based Protocols

• Pros
• Fairly deterministic (for a network)
• Cons
• Not as widely available
• Latencies can be high (according to some)

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Token-Based Protocols (cont.)

• FDDI (Fiber Distributed Data Interface)
• Uses token-based, ring topology
• Data rates equivalent to Fast Ethernet
• Fault tolerant
• RTFC (RT Fiber Communications)
• Not much recent work on IEEE 802.5

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Scheduling
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Scheduling

• Packet scheduling
• Task scheduling
• Resource scheduling

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Packet Scheduling

• Vital for RT communications
• Some probabilistic work
• Some work with co-scheduling RT and non-RT packets

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Packet Scheduling (cont.)

• Concatenated Hybrid Automatic Repeat Request
  (Uhlemann et al.)
• Repeated transmissions
• Signal averaging
• (m, k) scheduling (Hamdaoui and Ramanathan)
• Some work on a distance-based priority scheme to
  guarantee this
• Extended to multi-hop networks

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Packet Scheduling (cont.)

• Combining RT traffic with non-RT traffic
  (Chakraborty, Gries, and Thiele)
• Use Earliest Deadline First (EDF) scheduling
• Assign deadlines so that RT NRT traffic can
  coexist
• Claimed 25-45 improvement for NRT traffic

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Task Scheduling

• Landmark paper Liu Layland (1973)
• Analyzed static and dynamic scheduling (single
  processor)
• Many efforts to extend this
• Distributed Generalized Multiframe (DGMF) model
  (Chen, Mok, and Baruah)
• Presents new analytical framework

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Task Scheduling (cont.)

• Breaking tasks into subtasks (Kao and
  Garcia-Molina)
• Setting subtask deadlines
• Methodologies, performance issues

Task
Subtask
Sub-task
Subtask
Subtask
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Task Scheduling (cont.)

• Resource fragmentation (Bestavros)
• Load balancing
• Potentially bad for CPU-intensive jobs

Example
70
60
50
CPU 1
CPU 2
CPU 3
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Sample Systems
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Sample DRT Systems

• Everyone cites the same examples
• Air-traffic control, factory automation, nuclear
  power plants, etc.
• Difficult to find much info about these topics!
• Some work from avionics
• Possibly some air-traffic systems (FAST?)

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Summary
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Distributed RT Research Focus

• Networking
• Extending Ethernet to RT apps
• Packet scheduling issues
• Working with soft RT systems
• Leads to probabilistic techniques

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To-do Remaining Efforts

• More detail on resource usage, scheduling
• RT evaluation of token-based networks
• Find and examine sample implementation

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Questions/Suggestions?