Dynamic Soft Real Time Scheduling DSRT System

1
Dynamic Soft Real Time Scheduling (DSRT) System

• Klara Nahrstedt
• cs589kn

2
Problem Statement

• Multimedia applications need guaranteed and
  continuous processor time allocation.
• A MPEG decoder running at 10 frames per second.
• A game animation updating the movements of
  objects every 50ms.
• Applications have Soft Deadlines.
• Run on General Purpose OS.
• Limitations I/O Interrupts, Layered memory
  subsystem, priority inversion, ...

3
DSRT Solution

• Basic Features are insufficient.
• Unique/Advanced Features
• CPU Service Classes.
• Probing Service.
• Adaptive Service.
• Advanced Reservations.
• Security/Access Model.
• Multiprocessor Support.
• Distributed Monitoring Support.
• Middleware Implementation.

4
DSRT Concepts

• CPU Service Classes
• Mapping CPU Service Classes into a Multiprocessor
  Partitioning Design
• Execution Flow of a SRT Process

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CPU Service Classes
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Periodic Constant Processing Time Class

• Example Usage Pattern

Peak Processing Time10ms Period 100ms
0
10
110
210
100
200
Time(ms)
7
Periodic Variable Processing Time Class

• Example Usage Pattern

Period 100ms Peak Processing Time
30ms Sustainable Processing Time 15ms Burst
Tolerance 7ms
Time(ms)
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Multiprocessor Partitioning Design
Guaranteed Part
Non-guaranteed Part
PCPT Processes
Reserved Run
Overrun
PVPT Processes
TS Processes
Burst
Reserved Run
Overrun
RT Scheduler
Overrun Scheduler
TS Scheduler
Processor 1
RT Partition
Overrun Partition
TS Partition
Processor 2
RT Partition
Overrun Partition
TS Partition
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Execution Flow of a SRT Process
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Smart Probing (1)

• Goal Extract a reservation.
• Determine the most suitable Service Class and
  Parameters.
• Avoid over/under reserve resources.
• Needed because
• Processor usage is hardware platform dependent.
• Processor usage is input dependent.

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Smart Probing (2)

• DSRT runs a few iterations of SRT applications
  without reservation.
• DSRT monitors the usage iteration by iteration.
• DSRT analyzes the usage history.

Estimate a Reservation
Processor Usage
Conformance Test ?
Iteration Number
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Execution Flow of a DSRT Process
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Conformance Test (1)
Processor Usage
Nonconformance
Conformance
Burst Tolerance
Reserved Usage
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Conformance Test (2)

• SSBTR System Specific Burst Tolerance Rate.

PVPT Class
PCPT Class
Processor Usage
Processor Usage
SPT(1SSBTR)BT
PPT(1SSBTR)
PPT(1SSBTR)
SPT Leak every P
PPT Leak every P
PPT Leak every P
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Smart Probing (3)

• Compute average processor usage 50ms
• Compute peak processor usage 62ms.
• Max Burst 12ms

50ms
40ms
62ms
43ms
55ms
50ms
50ms
50ms
50ms
50ms
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Smart Probing (4)

• SSBTR 10
• Average Processor Usage SSBTR 5ms
• If Max Burst lt 5ms
• Constant Processing Time Class
• else if Max Burst (12ms) gt 5ms,
• Variable Processing Time Class
• Sustainable Processing Time 50ms
• Peak Processing Time 62ms
• Burst Tolerance 12ms - 5ms 7ms

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Execution Flow of a DSRT Process
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Admission Control Test

• Given a reservation request, determine
• Resource Availability. (1) (2)
• Processor Binding. (2)
• Preemptive Earliest Deadline First

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Execution Flow of a DSRT Process
Adjusted Contract
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Partition Scheduling
Top-Level Scheduler
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RT Partition Scheduler
Processor 1
Waiting Queue (Sorted with the earliest
released time)
p
(2a) finished one iteration
p
(3) released for next iteration
(1) admitted
p
Runnable Queue (Sorted with the earliest
deadline)
(2b) overrunning
Overrun Partition Queues
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Overrun Partition Scheduler
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Execution Flow of a DSRT Process
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Adaptation
Scene 2
Scene 1

• Adjust Reservation.
• 3 Strategies
• Exponential Average
• Range
• Statistical

Frame 275
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Exponential Average Adaptation Strategy

• Specification
• Window Size (ws).
• Alpha (?)
• Xi Guaranteed Parameter in a reservation.
• Xi-1 Actual Usage.

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Statistical Adaptation Strategy

• Specification
• Window Size (ws).
• Overrun Tolerance Frequency (f).
• Example ws10, f20. Adjust X40ms.

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Implementation Building Blocks

• A Real Time Timer.
• setiimer() in Solaris and Irix. Lots of Timers in
  NT.
• Fixed Real Time Priority.
• priocntl() in Solaris, sched_setscheduler() in
  Irix, setPriorityClass() in NT.
• Processor Affinity.
• processor_bind() in Solaris, sysmp(MP_MUSTRUN_PID)
  in Irix, and setProcessAffinityMask() in NT.

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User-level Priority Dispatch
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C APIs

• CpuApi cpu
• CpuReservation reservation
• // Probing Phase
• cpu.probe()
• cpu.start()
• for (int i0 iltnumProbeIterations i)
• doJob()
• cpu.yield()
• cpu.stop()
• cpu.probeEnd()
• cpu.probeMatch(reservation)

// Reservation Phase cpu.reserve(reservation) cpu
.setAdaptStrategy(strategy) // Execution
Phase cpu.start() for () doJob() cpu.yield
() cpu.stop() cpu.free()
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Experiment

• Dual Processor Sun Ultra 2 Machine.
• Solaris 2.6 Operating System.
• Dispatch Latency 400us. Includes
• Scheduling Algorithm.
• Context Switches.
• Various system calls.

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Experiment Setup

• Run 8 TS processes and 5 SRT processes
  concurrently.
• TS1-6 Computational intensive programs.
• TS7-8 Compilation programs.
• SRT1 A MPEG player at 10 FPS.
• Probing (PVPT class, P100ms,
• SPT28ms, PPT40ms, BT11ms).
• Adaptation Strategy (Statistical,
• f 20, ws 20).

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Experimental Setup (Cont.)

• SRT2 A MPEG player at 20 FPS.
• Probing (PVPT class, P50ms,
• SPT14ms, PPT21ms, BT6ms)
• SRT3 A sampling program.
• Probing(PCPT class, P50ms,
• PPT10ms)
• SRT4 A Java RocksInSpace game.
• (PCPT class, P100ms, PPT30ms).
• SRT5 Misbehaving greedy program.
• (PCPT class, P500ms, PPT10ms).

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Experimental Result
SRT2 MPEG player with P50ms.
SRT1 MPEG player With P100ms
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Experimental Result (Cont.)
SRT3 Sampling program with P50ms.
SRT4 Java game with P100ms.
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Conclusion

• Multimedia needs strong support from the OS via
  scheduling policies, differentiation policies and
  mechanisms
• Soft real-time guarantees can be made using
  priority mechanisms, but in a careful manner
• Changes can be made via middleware systems or
  directly inside of kernels, but there are
  trade-offs