1. Effects of Clock Resolution on the Scheduling of Interactive and Soft Real- Time Processes by Yoav Etsion, Dan Tsafrir, Dror G. Feitelson Presented by James Volpe

2. 2 Key Points Modern interactive applications not supported in commodity operating systems Instead of specialized APIs, tune commodity operating systems Clock interrupt rates unchanged last 30 years Increasing clock interrupt rate can help Additional overhead is acceptable

3. 3 Real-time Problem Xine MPEG player Short clip of 500 frames

4. 4 Process Scheduling run queue task_sched sleep queue process xx waiting for IO process xy waiting for time jiffies to display frame process xz waiting for network if another resource required, back to sleep queue single CPU if time slice complete and process not finished then back to run queue Execute for every interrupt Increase jiffies Bill process using CPU Adjust priorites Check task priorities etc... * modified from Simone Demblon,Sebastian Spitzner 100 Hz

5. 5 Process Scheduling Cont. Commodity operating systems use priority- based scheduling Static and dynamic priorities High CPU usage, the lower the priority Conflict with modern multimedia applications

6. 6 Real-time Problem Up Close

7. 7 Related Work RT-Linux, one-shot timers, soft timers, firm timers and priority adjustments Require special APIs and/or non-trivial modifications to the system Why not increase the clock interrupt rate and see what happens?

8. 8 Motivation 100 Hz clock interrupt rates  10 MHz CPU 100,000 instructions/interrupt  1 GHz CPU 10,000,000 instructions/interrupt Clock interrupt rate has become too coarse  Two orders of magnitude of additional instructions per interrupt

9. 9 Benefits of High Tick Rate Take advantage of today’s faster processors Better timing for meeting deadlines Accurate billing of processes

10. 10 Test Platform Pentium 90 to Pentium-IV 2.4 GHz Measurements done on 664 MHz Pentium III Linux kernel 2.4.8 (RedHat 7.0) Klogger used for logging scheduling related events: context switching, recalculation of priorities, forks, execs Workload from Emacs, Xine, Quake 3, CPU- bound processes

11. 11 Timing Advantage

12. 12 Timing Advantage

13. 13 Billing Advantage Application Billing ratioMissed quanta @100Hz@1000Hz@100Hz@1000Hz Emacs1.07460.946895.96%73.42% Xine1.27501.024989.46%74.81% Quake1.03101.033754.17%23.23% X Server o 0.02020.931999.43%64.05% CPU-bound1.00711.00437.86%7.83% CPU+Quake1.03331.039026.71%2.36% o When running Xine Table 3. Scheduler billing success rate

14. 14 Billing Advantage Cont.

15. 15 The Cost Is Overhead More clock interrupts increases overhead Time needed to process interrupts Higher number of context switches Operating systems become less efficient as clock rate increases

16. 16 Clock Interrupt Overhead Do operating systems become as fast as hardware? Table 1. Interrupt processing overheads on different processor generations Processor DefaultWithout 8253 Cyclesμsμs μsμs P-90814±1809.02498±4665.53 PP-2001654±5538.31462±7622.32 PII-3502342±3036.71306±3110.88 PIII-6643972±4625.98327±4870.49 PIII-1.1336377±6025.64426±9140.38 PIV-2.414603±4366.11445±5500.19 A1.46710494±3967.15202±4610.14

17. 17 Clock Interrupt Overhead Cont. Do operating systems become as fast as hardware? Processor Context switchCache BW MB/s Trap Cyclesμsμs μsμs P-901871±65620.7528±1153±241.70 PP-2001530±3897.69705±26379±751.91 PII-3501327±3313.801314±29343±680.98 PIII-6641317±4241.982512±32348±1630.52 PIII-1.1331330±4411.184286±82364±2780.32 PIV-2.43792±8571.593016±471712±320.72 A1.4671436±4770.983962±63274±200.19 Table 2. Other overheads on different processor generations

18. 18 Clock Interrupt Overhead Cont. Overhead from sorting an array with 50ms quantum

19. 19 Clock Interrupt Overhead Cont. Overhead from sorting an array with quantum equal to 5 ticks

20. 20 Conclusion Increasing clock interrupt rate to 1000 Hz achieves timing and billing advantages Overhead is minimal Suggest making a settable parameter instead of compiled constant

21. 21 Evaluation Results are promising Production level example would strengthen argument Changing clock interrupt rate in Linux not an end user job Testing is needed for implementation

22. 22 Questions / Discussion