1. Applying kernel timers Could we prevent a device-driver from consuming an excessive amount of processor time?

2. Degraded system performance Recall our ‘Interrupt Activity Monitor’ demo It’s posted on class website (‘activity.cpp’) Repeatedly called a driver’s ‘read’ function But used over 99-percent of the cpu time! (We discovered that fact with ‘top’ utility) Name of the driver module was ‘activity.c’

3. Kernel software timers We also learned how to use kernel timers An easy-to-use programming interface Data-object: struct timer_list; And four kernel functions: init_timer(); add_timer(); mod_timer(); del_timer();

4. Sleep/Wakeup Earlier we encountered Linux task ‘states’ A Linux task can easily be ‘put to sleep’ While asleep a task doesn’t use cpu time A timer-function could ‘wake up’ our task

5. How it might work Our device-driver’s ‘read()’ functon: –Before reading any new data, check to see how long it’s been since we last did a ‘read’ –If we discover we read data quite recently, then we could set-a-timer and go-to-sleep –When awakened we’ll return the new data This proposed scheme is like a ‘throttle’

6. What does timer-function do? Our driver needs a ‘ready’ flag: 0 or 1 The ‘read()’ function sets: ready = FALSE The timer-function sets: ready = TRUE ‘read()’ will sleep until: ready == TRUE And timer-function will ‘wake up’ driver

7. Let’s try it! Some demo-code is on our class website: –Device-driver module: ‘tsc.c’ –Application program: ‘watchtsc.cpp’ First: Try out the demo (use ‘top’ utility) Next: Modify the driver-module –Add a wait-queue –Add a kernel timer Finally: Use ‘top’ again to check cpu usage

8. TimeStamp Counter Pentium cpu has a built-in TSC register Incremented once every cpu clock-cycle Register is 64-bits wide (8-bytes) It won’t overflow for at least a decade! Accessed using special instruction: ‘rdtsc’ EAX=least significant 32-bits of TSC EDX=most signigicant 32-bits of TSC