Applying kernel timers
Could we prevent a device-driver
from consuming an excessive
amount of processor time?
Degraded system performance
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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’
Kernel software timers
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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();
Sleep/Wakeup
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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
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’
What does timer-function do?
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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
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
TimeStamp Counter
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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