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School of Computer Science & Engineering
University of New South Wales
 Advanced Operating Systems 
 COMP9242 2002/S2 

Cool Stuff: Soft Timers[AD99]

Problem: want high-resolution timers

E.g.: Timeouts in L4, prefer high resolution.

• Kernel has wakeup queues
• Kernel sets timer interrupt at certain frequency.
• At each interrupt (tick) checks wakeup queue.
• Handling interrupts is costly.
• Handling interrupt pollutes cache.
• Cannot have very high frequency timer ticks.

What can we do?

Idea: soft timers rather than hardware interrupt

• Check wakeup queues when cost of handling with them is low.
• This is generally the case when in the kernel already.
• Do it at a time where minimal state needs to be saved.
• Suitable times:
  • just before returning from system call,
  • at the end of exception handling,
  • at the end of interrupt handling,
  • in idle loop.

Limitations of soft timers:

• No absolute, only probabilistic accuracy.
• Actual accuracy depends on frequency of trigger events.
• Can enforce absolute upper bound on delay via timer tick.