Advanced Operating Systems   COMP9242 2002/S2

UNSW

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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.