Clock Sources, Timing Accuracy, and Hardware Considerations

timer selected during boot: <screen> # dmesg | grep "Timecounter" Timecounter "ACPI-fast" frequency 3579545 Hz quality 900 Timecounter "i8254" frequency 1193182 Hz quality 0 Timecounters tick every 10.000 msec Timecounter "TSC" frequency 2531787134 Hz quality 800 # sysctl kern.timecounter.hardware=TSC kern.timecounter.hardware: ACPI-fast -> TSC </screen> </para> <para> Other systems may only allow setting the time source on boot. On older Linux systems the "clock" kernel setting is the only way to make this sort of change. And even on some more recent ones, the only option you'll see for a clock source is "jiffies". Jiffies are the older Linux software clock implementation, which can have good resolution when it's backed by fast enough timing hardware, as in this example: <screen><![CDATA[ $ cat /sys/devices/system/clocksource/clocksource0/available_clocksource jiffies $ dmesg | grep time.c time.c: Using 3.579545 MHz WALL PM GTOD PIT/TSC timer. time.c: Detected 2400.153 MHz processor. $ pg_test_timing Testing timing overhead for 3 seconds. Per timing duration including loop overhead: 97.75 ns Histogram of timing durations: < us % of total count 1 90.23734 27694571 2 9.75277 2993204 4 0.00981 3010 8 0.00007 22 16 0.00000 1 32 0.00000 1 ]]></screen></para> </refsect2> <refsect2> <title>Clock Hardware and Timing Accuracy</title> <para> Collecting accurate timing information is normally done on computers using hardware clocks with various levels of accuracy. With some hardware the operating systems can pass the system clock time almost directly to programs. A system clock can also be derived from a chip that simply provides timing interrupts, periodic ticks at some known time interval. In either case, operating system kernels provide a clock source that hides these details. But the accuracy of that clock source and how quickly it can return results varies based on the underlying hardware. </para> <para> Inaccurate time keeping can result in system instability. Test any change to the clock source very carefully. Operating system defaults are sometimes made to favor reliability over best accuracy. And if you are using a virtual machine, look into the recommended time sources compatible

Different systems allow setting the time source, sometimes only on boot. Older Linux systems use the 'clock' kernel setting, while some recent ones may only offer 'jiffies,' an older software clock implementation. Accurate timing relies on hardware clocks, where OS kernels provide clock sources, but their accuracy varies based on hardware. Inaccurate timekeeping can cause system instability. OS defaults sometimes prioritize reliability over accuracy, and virtual machines require compatible time sources.