Supporting Time-Sensitive Applications on a GENERAL PURPOSE ( Commodity ) OS

1. Supporting Time-Sensitive Applications on a GENERAL PURPOSE (Commodity) OS Ashvin Goel, Luca Abeni, Charles Krasic, Jim Snow, Jonathan Walpole Dept of Computer Science , OGI Presenter – PRADEEP ANKALA CS533

2. Real-Time & General Purpose OS’s - Real-Time OS: VxWorks, QNX, LynxOS, eCos,DeltaOS, PSX,embOS, ... - GPOS: no support for real-time - applications, focus on ‘fairness’. - BUT, people love GPOSs, e.g., Linux: CS533

3. Outline • Introduction • Implementing Time-Sensitive Linux • Experiment • Conclusions CS533

4. Introduction • Time-sensitive Applications ( Ex:multimedia, soft real-time applications. ) • High precision timing facility • Well designed preemptible kernel • Appropriate scheduling technique • Time-Sensitive Requirement • Timing Mechanism • Responsive Kernel • CPU Scheduling Algorithm CS533

5. Paper’s Contribution • 􀁺 Situation: Linux (and many other OS’s) cannot handle real-time applications • 􀁺 Hypothesis: a few techniques can overcome the limitations of Linux – Firm timer (high resolution) – Fine-grain kernel preemption – Priority and reservation-based scheduling • 􀁺 Good performance and low overhead CS533

6. Problems and Solutions • 􀁺 Firm Timer reduces timer latency – Low overhead important • 􀁺 Fine-grain kernel preemption reduces preemption latency – Responsive kernel • 􀁺 Good schedulers reduce scheduling latency – Proportion-period scheduler CS533

7. Kernel Latency CS533

8. TSL Introduction • Integrated techniques of TSL • Firm timers • One-shot timers • Soft timers • Timer Overshoot • Fine-grained kernel preemptibility • CPU Scheduling • Priority-based • Proportion-period CS533

9. Firm Timers • One-shot timers - Reprogrammed for next timer interrupt • + Soft timers (reduce interrupt overhead) – Polling at convenient times (e.g., syscall return) (No interrupt, check timer at some check pointers Reduce # of timer interrupts Reduce # of context switches) • +Timer Overshoot – Reduce variance due to soft timer uncertainty • Combination of One-shot timers and Soft timers with system wide timer overshoot parameter CS533

10. Fine-Grained Kernel Preemptibility Non-preemptible kernel • Sometime interrupt will be deferred • Interrupt is disabled In the critical section Preemption latency under Linux is greater 30ms  Preemptiable kernel • Preemptive anytime when kernel is not accessing shared data • Use spinlock to protect shared data Preemption latency is depend on the max time for which a spinlock is held inside the kernel CS533

11. Fine-Grained Kernel Preemptibility( cont) • What is a Responsive Kernel? Answer: Preemptible kernel + Explicit preemption • Explicit insertion of preemption point at strategic points inside the kernel • Allow preemption anytime the kernel is not accessing shared data structure • Releasing spin-locks at strategic points in long code sections CS533

12. CPU Scheduling • Proportion-Period CPU Scheduling • Provide temporal protection • Priority CPU Scheduling • Priority inversion • Highest locking priority protocol (HLP) CS533

13. Experiment • Evaluate • The behavior of time-sensitive applications running on TSL • The overheads of TSL • Environment : • 1.5 GHz Pentium-4 Intel processor with 512 MB memory • Linux kernel 2.4.16 CS533

14. Latency in Real Applications(mplayer – a open-source audio/video playerProportion-period scheduler - a kernel-level “application”) Non-kernel CPU load Kernel CPU load File System Load CS533

15. Non-kernel CPU Load CS533

16. Kernel CPU Load CS533

17. File System Load CS533

18. Proportion-Period Scheduler CS533

19. System Overhead • Executing code at the newly inserted preemption points • Checking preemption • Executing preemption if needed • Executing firm timers CS533

20. Checking for Preemption • Memory test • Sequentially access a large array • Fork test • Create 512 processes as soon as possible • File-system test • Repeatedly copy data from a 2MB buffer to 8 MB file CS533

21. Firm Timers • Costs associated with hard timers exclusively • Interrupt handling and cache pollution • Costs that hard and soft timers have in common • Handling timers and the executing preemption for an expired timer thread • Costs associated with soft timers exclusively • Checking for soft timers CS533

22. Firm Timers CS533

23. Firm Timers CS533

24. Firm Timers CS533

25. Firm timers Discussion -Firm timers lower overhead when soft-timer checks find timers -Firm timers higher overhead when soft-timer checks find nothing and timer goes off -From their work, firm timers lower when more than 2.1% of timer checks find timer CS533

26. Firm Timers Discussion ( cont) • Nt = Nh+Ns • Total cost = CcNc+ChNh+CsNs • Pure hardware = ChNt • Total cost < Pure hardware=>Ns/Nc > Cc/(Ch-Cs) CS533

27. Conclusions • TSL can support applications needing fine-grained resource allocation and low latency response  • Variations of less than 400 microseconds under heavy CPU and file system load  • Overhead is low • Ns/Nc > K condition under which firm timers are effective CS533

28. Thanks for YOUR PATIENCE In LISTENING    CS533