An Overview of Distributed Real-Time Systems Research

1. An Overview of Distributed Real-Time Systems Research By Brian Demers March 24, 2003 CS 535, Spring 2003

2. 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

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

5. Networking

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

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

8. Halmsted University Hardware solution: Propose a network connected exclusively by switches (eliminating collisions) Switches perform data buffering, manage real-time streams Ethernet Improvements Switch

9. 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 Ethernet Improvements (cont.) Higher Network Layers Regulator Data-link Layer Traffic smoothing approach

10. Token-Based Protocols • Pros • Fairly deterministic (for a network) • Cons • Not as widely available • Latencies can be high (according to some)

11. 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

12. Scheduling

13. Scheduling • Packet scheduling • Task scheduling • Resource scheduling

14. Packet Scheduling • Vital for RT communications • Some probabilistic work • Some work with co-scheduling RT and non-RT packets

15. 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

16. 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

17. 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

18. Breaking tasks into subtasks (Kao and Garcia-Molina) Setting subtask deadlines Methodologies, performance issues Task Scheduling (cont.) Task Subtask Sub-task Subtask Subtask

19. Resource fragmentation (Bestavros) Load balancing Potentially bad for CPU-intensive jobs Task Scheduling (cont.) Example: 70% 60% 50% CPU 1 CPU 2 CPU 3

20. Sample Systems

21. 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?)

22. Summary

23. Distributed RT: Research Focus • Networking • Extending Ethernet to RT apps • Packet scheduling issues • Working with “soft” RT systems • Leads to probabilistic techniques

24. To-do: Remaining Efforts • More detail on resource usage, scheduling • RT evaluation of token-based networks • Find and examine sample implementation

25. Questions/Suggestions?