Dreams in a Nutshell

1
Dreams in a Nutshell

• Steven Sommer
• steve_at_mpce.mq.edu.au

Microsoft Research Institute Department of
Computing Macquarie University
2
Overview

• 1. Background
• 2. Dreams Model
• 3. Overrun
• 4. Implementation

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Dreams

• Distributed Real-time Extensions with Application
  to Multimedia Systems
• Extend conventional operating systems to support
  distributed real-time applications
• Paradigm preserving

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Key Paradigm Differences

• Multiple competing applications
• Temporal protection
• Absence of a priori knowledge
• Different kinds of applications
• Subsystems (servers)
• Interrupts
• Dynamic distribution

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2. The Dreams Model

• Transient Periodic Processes
• Periodic Invocations
• Specified
• Period
• Deadline
• Execution Time

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The Dreams Model
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3. Overrun Concepts

• System guarantee
• A real-time task will receive its reserved
  resources between each invocations start-time
  and deadline
• A task which consumes its reserved capacity
  without completing has overrun
• Only overrun tasks can miss their deadlines
• Task continues in overrun state
• Overrun tasks receive refreshed resources at the
  start of their next period

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Scheduling and Overrun
Waiting
Invocation Completion
Invocations Start Time
Scheduled Thread
Runnable
Invocation Overrun
Overrun Try
Overrun
Overrun Leave
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Sample Schedule
Process Period Reservation Av. Execution
Time first scheduled T1 100 20
20 70 T2 250 110
100 70 T3 1000 440
400 480
. 10ms
0 100 200 300
400 Time ID 01234567890123456789012345678901234567
890123456789 ---- 4500 1 .......RC........RC.....
...RC........RC........R1O 4500 2
.......SR2222222C...............R22222R22C........
4500 3 33333333........R3R333333333R3333........
R333333.R 5000 1 .......C.........R1O..RC...R1C.
......R1O..C....R1O 5000 2 .......R2222222C......
..........R22222.R22C....... 5000 3
33333333.......R33.R33C.........................SR
5500 1 .......C.........R1O.......C.........R1O
.......C.. 5500 2 .......R2222222C...............
.R22222.R22C....... 5500 3 33333333.......R33.R33
33333R33333.........R3333R33
Legend(S)tart(time) (R)un (scheduled)
(C)ompleted (O)verrun Multiple
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4. Implementation

• Windows NT 3.51 implementation
• Minimal impact on
• Non-real-time execution
• Existing operating system code
• Programming paradigm

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Dreams Client
Win32 Client
Applications
Protected subsystems (servers)
Dreams Subsystem
Win32 Subsystem
User mode
Kernel mode
Executive
System services
I/O manager
Virtual Memory Manager
Security Reference Monitor
Local Procedure Call
Object Manager
Process Manager
File systems
Cache manager
Device drivers
Kernel
Network drivers
Hardware abstraction layer
Message Passing / Shared Memory
System Trap
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Dreams Subsystem Memory Space
Dreams Client Application Memory Space
Reservation Manager
Application Process
Real-Time Thread Manager
API Call
Dreams DLL
Procedure call via named pipes
Blocking Access
Non real-time priority
Shared Data Area Requests / Process Table
Create Thread
Real-time priority
Non blocking access
Real-Time Thread
Real-Time Scheduler Enforcer
Non blocking communication
API Call
Periodic Invocation
Dreams DLL
Shared Memory
Kernel Call
Window NT Kernel
Scheduling via NT Priority Control
Kernel Mode Dreams
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Implementation Experience

• Dreams extensions in a subsystem
• Meet implementation goals
• Simpler development, modification, and testing
• Priority inheritance
• Useful in all operating systems (not just
  real-time)
• Performance improvement
• Two tiered scheduler (with priority inheritance)
• Hides scheduling complexities
• Simpler scheduling
• Simpler schedulability model
• Simpler timing and enforcement

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Summary

• Background
• Dreams Model
• Overrun
• Implementation

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Sample Schedule
Process Period Reservation Av. Execution
Time first scheduled T1 100 20
20 70 T2 250 110
100 70 T3 1000 440
400 480
. 10ms
0 100 200 300
400 Time ID 01234567890123456789012345678901234567
890123456789 ---- 4500 1 .......RC........RC.....
...RC........RC........R1O 4500 2
.......SR2222222C...............R22222R22C........
4500 3 33333333........R3R333333333R3333........
R333333.R 5000 1 .......C.........R1O..RC...R1C.
......R1O..C....R1O 5000 2 .......R2222222C......
..........R22222.R22C....... 5000 3
33333333.......R33.R33C.........................SR
5500 1 .......C.........R1O.......C.........R1O
.......C.. 5500 2 .......R2222222C...............
.R22222.R22C....... 5500 3 33333333.......R33.R33
33333R33333.........R3333R33
Legend(S)tart(time) (R)un (scheduled)
(C)ompleted (O)verrun Multiple