Dreams in a Nutshell
Steven Sommer
steve@mpce.mq.edu.au
Microsoft Research Institute
Department of Computing
Macquarie University
Overview
1. Background
2. Dreams Model
3. Overrun
4. Implementation
August 1997
USENIX Windows NT Workshop
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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
August 1997
USENIX Windows NT Workshop
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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
August 1997
distribution
USENIX Windows NT Workshop
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2. The Dreams Model
 Transient
Periodic Processes
 Periodic
Invocations
 Specified
Period
 Deadline
 Execution Time

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USENIX Windows NT Workshop
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Application
100%
Free
Create
Real-Time
Process
The Dreams Model
Make
Reservation
Schedulability Test
Reservations
Manager
Distribute
Yes /
No
Create the
Process
0%
Real-Time
Ready-to-Run Queue
Process Table
Reserved
Process
Manager
P5
P1
P9
System
Scheduler
CPU
80486
P1
P5
P9
Allocate to CPU
P1
Add
Process
P9
P5
Enforcer
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3. Overrun Concepts
 System
guarantee:
 A real-time
task will receive its reserved resources
between each invocation’s 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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USENIX Windows NT Workshop
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Scheduling and Overrun
Waiting
Invocation
Completion
Invocation’s
Start Time
Runnable
Scheduled Thread
Invocation
Overrun
Overrun Try
Overrun
Overrun Leave
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Sample Schedule
Process
T1
T2
T3
Period Reservation Av. Execution Time first scheduled
100
20
20
~70
250
110
100
~70
1000
440
400
~480
. = 10ms
0
100
200
300
400
Time ID 01234567890123456789012345678901234567890123456789
---4500 1 .......RC........RC........RC........RC........R1O
4500 2 .......SR2222222C...............R22222R22C........
4500 3 33333333........R3R333333333R3333........R333333.R
5000
5000
5000
1 .......C.........R1O..RC...R1C.......R1O..C....R1O
2 .......R2222222C................R22222.R22C.......
3 33333333.......R33.R33C.........................SR
5500
5500
5500
1 .......C.........R1O.......C.........R1O.......C..
2 .......R2222222C................R22222.R22C.......
3 33333333.......R33.R3333333R33333.........R3333R33
Legend:(S)tart(time) (R)un (scheduled) (C)ompleted (O)verrun
Multiple
4. Implementation
 Windows
NT 3.51 implementation
 Minimal impact on:
 Non-real-time
execution
 Existing operating system code
 Programming paradigm
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Win32™
Client
Dreams
Client
Dreams
Subsystem
Win32
Subsystem
OS/2®
Client
Applications
OS/2
Subsystem
Protected
subsystems
(servers)
POSIX
Client
POSIX
Subsystem
User mode
Executive
Kernel mode
System services
Object
Manager
Security
Reference
Monitor
Process
Manager
Local
Procedure
Call
Virtual
Memory
Manager
Kernel
I/O manager
File systems
Cache manager
Device drivers
Network drivers
Hardware abstraction layer
Message Passing / Shared Memory
System Trap
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Dreams Client Application
Memory Space
Dreams Subsystem
Memory Space
Application
Process
Real-Time
Thread
Manager
Reservation
Manager
API Call
Dreams DLL
Create Thread
Procedure call
via named
pipes
Dreams DLL
Shared Data Area
Requests / Process Table
Non blocking access
Real-Time
Thread
API Call Periodic Invocation
Blocking Access
Non blocking
communication
Shared Memory
Non real-time
priority
Real-time
priority
Real-Time
Scheduler
+
Enforcer
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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