Chapter 15. Managing system clocks to satisfy application needs

Procedure

1. View the available clock sources.

   # cat /sys/devices/system/clocksource/clocksource0/available_clocksource
   tsc hpet acpi_pm

   As an example, consider the available clock sources in the system are TSC, HPET, and ACPI_PM.

2. Write the name of the clock source you want to use to the /sys/devices/system/clocksource/clocksource0/current_clocksource file.

   # echo hpet > /sys/devices/system/clocksource/clocksource0/current_clocksource

   Note

   The changes apply to the clock source currently in use. When the system reboots, the default clock is used. To make the change persistent, see Making persistent kernel tuning parameter changes.

Procedure

1. Change to the directory in which the clock_timing program is saved.

   # cd clock_test

2. View the available clock sources in your system.

   # cat /sys/devices/system/clocksource/clocksource0/available_clocksource
   tsc hpet acpi_pm

   In this example, the available clock sources in the system are TSC, HPET, and ACPI_PM.

3. View the currently used clock source.

   # cat /sys/devices/system/clocksource/clocksource0/current_clocksource
   tsc

   In this example, the current clock source in the system is TSC.

4. Run the time utility in conjunction with the ./clock_timing program. The output displays the duration required to read the clock source 10 million times.

   # time ./clock_timing
   
   	real	0m0.601s
   	user	0m0.592s
   	sys	0m0.002s

   The example shows the following parameters:

   • real - The total time spent beginning from program invocation until the process ends. real includes user and kernel times, and will usually be larger than the sum of the latter two. If this process is interrupted by an application with higher priority, or by a system event such as a hardware interrupt (IRQ), this time spent waiting is also computed under real.
   • user - The time the process spent in user space performing tasks that did not require kernel intervention.
   • sys - The time spent by the kernel while performing tasks required by the user process. These tasks include opening files, reading and writing to files or I/O ports, memory allocation, thread creation, and network related activities.

5. Write the name of the next clock source you want to test to the /sys/devices/system/clocksource/clocksource0/current_clocksource file.

   # echo hpet > /sys/devices/system/clocksource/clocksource0/current_clocksource

   In this example, the current clock source is changed to HPET.

6. Repeat steps 4 and 5 for all of the available clock sources.
7. Compare the results of step 4 for all of the available clock sources.

Procedure

1. Enable the clocksource=tsc and powernow-k8.tscsync=1 kernel options:

   # grubby --update-kernel=ALL --args="clocksource=tsc powernow-k8.tscsync=1"

   This forces the use of TSC and enables simultaneous core processor frequency transitions.

2. Restart the machine.

1. Create a directory for the program files.

   $ mkdir clock_test

2. Change to the created directory.

   $ cd clock_test

3. Create a source file and open it in a text editor.

   $ {EDITOR} clock_timing.c

4. Enter the following into the file:

   #include <time.h>
   void main()
   {
   	int rc;
   	long i;
   	struct timespec ts;
   
           for(i=0; i<10000000; i++) {
   		rc = clock_gettime(CLOCK_MONOTONIC, &ts);
   	}
   }

5. Save the file and exit the editor.

6. Compile the file.

   $ gcc clock_timing.c -o clock_timing -lrt

   The clock_timing program is ready and can be run from the directory in which it is saved.