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 one 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

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 real- time streams Switch

Ethernet Improvements (cont.) T raffic 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 Data-link Layer Regulator Higher Network Layers 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

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 Sub- task Subtask

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

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?