Applying kernel timers

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
• Its posted on class website (activity.cpp)
• Repeatedly called a drivers 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 doesnt use cpu time
• A timer-function could wake up our task

5
How it might work

• Our device-drivers read() functon
• Before reading any new data, check to see
• how long its 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 well 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
Lets 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 wont overflow for at least a decade!
• Accessed using special instruction rdtsc
• EAXleast significant 32-bits of TSC
• EDXmost signigicant 32-bits of TSC