Clock Hardware, Timing Accuracy, and Available Timers

</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 with it. Virtual hardware faces additional difficulties when emulating timers, and there are often per operating system settings suggested by vendors. </para> <para> The Time Stamp Counter (TSC) clock source is the most accurate one available on current generation CPUs. It's the preferred way to track the system time when it's supported by the operating system and the TSC clock is reliable. There are several ways that TSC can fail to provide an accurate timing source, making it unreliable. Older systems can have a TSC clock that varies based on the CPU temperature, making it unusable for timing. Trying to use TSC on some older multicore CPUs can give a reported time that's inconsistent among multiple cores. This can result in the time going backwards, a problem this program checks for. And even the newest systems can fail to provide accurate TSC timing with very aggressive power saving configurations. </para> <para> Newer operating systems may check for the known TSC problems and switch to a slower, more stable clock source when they are seen. If your system supports TSC time but doesn't default to that, it may be disabled for a good reason. And some operating systems may not detect all the possible problems correctly, or will allow using TSC even in situations where it's known to be inaccurate. </para> <para> The High Precision Event Timer (HPET) is the preferred timer on systems where it's available and TSC is not accurate. The timer chip itself is programmable to allow up to 100 nanosecond resolution, but you may not see that much accuracy in your system clock. </para> <para> Advanced Configuration and Power Interface (ACPI) provides a Power Management (PM) Timer, which Linux refers to as the acpi_pm. The clock derived from acpi_pm will at best provide 300 nanosecond resolution. </para> <para> Timers used on older PC hardware include the 8254 Programmable Interval Timer (PIT), the real-time clock (RTC), the Advanced Programmable Interrupt Controller (APIC) timer, and the Cyclone timer. These timers aim for millisecond resolution. </para> </refsect2> </refsect1> <refsect1> <title>See Also</title> <simplelist type="inline"> <member><xref linkend="sql-explain"/></member> </simplelist> </refsect1> </refentry>

Accurate timing relies on hardware clocks and OS-provided clock sources. The Time Stamp Counter (TSC) is most accurate but can be unreliable due to temperature variations, multicore inconsistencies, and power-saving configurations. Newer OSes may switch to slower, stable sources when TSC is problematic. The High Precision Event Timer (HPET) is preferred when TSC is inaccurate. Other timers include ACPI's Power Management Timer, offering limited resolution, and older PC hardware timers like the 8254 PIT and RTC, aiming for millisecond resolution.