1. Supporting Time-Sensitive Applications on a Commodity OSbyAshvin Goel, Luca Abeni, Charles Krasic, Jim Snow, Jonathan Walpole Jimi Watson

2. Why? • Time sensitive applications • Enhancing resource allocation • Throughput • Insignificant degradation

3. Four keys • Timing mechanisms • Timer latency • Responsive Kernel • Preemption latency • CPU scheduling algorithm • Scheduling latency • Use them all

5. Timers • Periodic timers • The old model • Shortest period 10msec • One-shot timers • Interrupts at specific instance • Reprogram • Soft timers • Handles events at soft timer points

6. Timers Comparison • Hard timers • Interrupt handling • Cache pollution • Soft timers • Polling for expired events • Firm timers • Better precision • Fewer interupts

7. Firm Timers • Combination of soft and one shot timers • Use soft timers to reduce overhead • Use one-shot timers to ensure events are handled • Overshoot

8. Example of overshoot 0 5 10 Time Last event handled Next event Overshoot

9. Firm timer implementation • Timer queue • APIC is set to next timer event plus global overshoot value • Possible to have overshoot for separate timers • Apps with tighter constraints incur penalty • Overhead is simply additional queue and overshoot • Periodic timers used for long timeouts • One shot timers only within 1 tick period • POSIX interface calls altered to take advantage of firm timers on TSL

10. Kernel preemptibility • May not be preemtible • Interrupts disabled • Executing in critical section • Solution • Shorten the length it is not preemptible • Explicit preemption points • Lock breaking preemtible kernel

11. Scheduling • Proportion-Period scheduling • Priority CPU scheduling

12. Proportion-Period scheduling • Temporal protection • Every time sensitive task requires specification of proportion and period • Proportion is how much CPU it needs • Period is how often the task needs CPU to guarantee proper function

13. Proportion Scheduling CPU CPU CPU CPU Time Periods

14. Priority scheduling • Priority assigned based on application needs • Run in the background • Priority inversion

15. Priority inversion

16. Testing • Compare Linux, soft timers, hard timers and firm timers • Multiple applications • Each require 10 msec intervals

17. 20 processes, 10 msec

18. 50 processes , 10 msec

19. 20 processes, 1 msec

20. Testing • Difference between hard and firm timers • Firm timers have lower overhead when a percentage of soft timers fire per poll • Ns/Nc > 2.1 % • Variation > 400 usec under heavy loads • Low overhead on throughput loads

21. Conclusions • Better performance for all applications • Gracefully handles soft real time apps • Very little impact on throughput • Useful general purpose OS