Augmented CPU Reservations Towards Predictable Execution on General-Purpose Operating System

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Augmented CPU ReservationsTowards Predictable
Execution on General-Purpose Operating System
RTAS 2001

• John Regehr, John A. Stankovic
• Department of Computer Science
• University of Virginia

Speaker MingChieh Chang
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Outline

• Background
• Open real-time systems
• Rez
• Stolen time
• Rez-C and Rez-FB
• Design
• Performance
• More stolen time data
• Related work
• Conclusions

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Support Soft Realtime in GPOS

• Reserved time for a thread periodically
• Reservation scheduler
• EXRez

Algorithm Base on EDF Each thread is assigned
a budget (e.g. time quantum) Decrease when the
thread runs Implementation In Windows 2000
kernel
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A Problem

• OS may steal time from the running AP and it is
  unaccountable
• DPCs in Windows NT / 2000
• Bottom half handlers in Unix

AP
User
OS kernel
Unpredictable penalty
ISR
HW
trigger
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Steal time Network Receive Processing
(expectation)
(Real)
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How to cope with???
Reserve more time
Augmented CPU reservation
Measure stolen time and compensate it
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Rez-C

• C ? Catch up
• When timer is expired
• Call getstolen( ) to get stolen time
• Set budget by stolen time
• Schedule by EDF

Compensate stolen time at this period
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Rez-FB (1/2)

• FB ? FeedBack
• Key idea
• Assume amount of stolen time is similar to the
  that of last period

consider stolen time at next period
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Rez-FB (2/2)
Set point R (requested reservation amount)
Actuator Ct (actual reservation amount)
Rez-FB
OS
Application
Feedback Pt (amount of CPU time that a thread
actually used)
Feedback equation Ct1 Ct G(R-Pt)
, G constant ?1
Evaluated each period for each reservation
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Experiment

• Machine
• P3-500
• 256 MB RAM
• Intel EtherExpress Pro/100B PCI Ethernet adapter
• OS
• Windows 2000 with Rez, Rez-C, Rez-FB
• Timer interrupt period
• 244us

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• Test Application (reserved time)
• Poll TSC (for measure)
• Period 20ms
• Reservation 4ms
• Workload (for stolen time)
• Receive processing for 100 Mbps Ethernet

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Augmented Reservation Performance
No workload
Because not consider hardware interrupt
(4m)
(3.6m)
(4.4m)
(4.8m)
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Performance of Rez-FB
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Overhead

• Compare with Windows 2000 Rez
• Rez-C
• 0.012 0.0028
• Rez-FB
• 0.017 0.0024

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Conclusion

• Stolen time is a key problem
• OSs have hundreds of drivers, many of which may
  steal time
• Augmented CPU reservations
• Simple
• Increase application scheduling predictability
  during stolen time