CS533 Concepts of Operating Systems

Class 18

Fine Grain Timing

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Questions

What is the main source of overhead for hard timers?

Why does it help to use soft timers?

How frequently do soft timers fire? o What if the application doesn't make any system calls?o What other kernel events can be used?o Can timer checks be inserted elsewhere??

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Questions

What is the main source of overhead for device interrupts?

o Can they be taken at more convenient times?o Can they be batched? o Why not turn off interrupts and just poll for I/O?

Why not batch timer events?o How can you take only the ones you need?o Why do soft timers help one shot timers?

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Questions

What is the main source of timing error in Linux?

Why hasn't timer frequency increased in line with CPU speed?

o how are the cost of interrupts related to the cost of synchronization instructions ?

o how will increased parallelism affect this problem?

Why does kernel preemptability affect timing accuracy?

o how can preemptability be improved?

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Questions

What is temporal protection?o Does proportion period scheduling support it?

Why might you need a feedback controller above a proportion period scheduler?

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For more information …

See the following papers available at:http://www.cs.pdx.edu/~walpole/

"A Measurement-Based Analysis of the Real-Time Performance of the Linux Kernel,"

Luca Abeni, Ashvin Goel, Buck Krasic, Jim Snow, and Jonathan Walpole, in Proceedings of The 8th IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS 2002), San Jose, California, September 2002.

"Real-Rate Scheduling," Ashvin Goel, Molly Shor and Jonathan Walpole, in Proceedings of the 10th IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS 2004), Toronto, Canada, May 2004.

"Operating System Support for Low-Latency Streaming"

Ashvin Goel, Ph.D. Thesis, OGI School of Science and Engineering, OHSU, July 2003.