An Overview of Distributed Real Time Systems Research

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

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

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

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

Networking

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

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

Ethernet Improvements • Halmsted University – Hardware solution: Propose a network connected exclusively by switches (eliminating collisions) – Switches perform data buffering, manage realtime streams Switch

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

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

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

Scheduling

Scheduling • Packet scheduling • Task scheduling • Resource scheduling

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

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

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

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

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

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

Sample Systems

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? )

Summary

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

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

Questions/Suggestions?