For me it is [Time Stamp Counter](https://en.wikipedia.org/wiki/Time_Stamp_Counter). As we may know from the [second part](https://0xax.gitbooks.io/linux-insides/content/Timers/timers-2.html) of this chapter, which describes internals of the `clocksource` framework in the Linux kernel, the best clock source in a system is a clock source with the best (highest) rating or in other words with the highest [frequency](https://en.wikipedia.org/wiki/Frequency).
Frequency of the [ACPI](https://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface) power management timer is `3.579545 MHz`. Frequency of the [High Precision Event Timer](https://en.wikipedia.org/wiki/High_Precision_Event_Timer) is at least `10 MHz`. And the frequency of the [Time Stamp Counter](https://en.wikipedia.org/wiki/Time_Stamp_Counter) depends on processor. For example On older processors, the `Time Stamp Counter` was counting internal processor clock cycles. This means its frequency changed when the processor's frequency scaling changed. The situation has changed for newer processors. Newer processors have an `invariant Time Stamp counter` that increments at a constant rate in all operational states of processor. Acutally we can get its frequency in the output of the `/proc/cpuinfo`. For example for the first processor in the system:
Frequency of the [ACPI](https://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface) power management timer is `3.579545 MHz`. Frequency of the [High Precision Event Timer](https://en.wikipedia.org/wiki/High_Precision_Event_Timer) is at least `10 MHz`. And the frequency of the [Time Stamp Counter](https://en.wikipedia.org/wiki/Time_Stamp_Counter) depends on processor. For example On older processors, the `Time Stamp Counter` was counting internal processor clock cycles. This means its frequency changed when the processor's frequency scaling changed. The situation has changed for newer processors. Newer processors have an `invariant Time Stamp counter` that increments at a constant rate in all operational states of processor. Actually we can get its frequency in the output of the `/proc/cpuinfo`. For example for the first processor in the system:
This is the end of the sixth part of the [chapter](https://0xax.gitbooks.io/linux-insides/content/Timers/index.html) that describes timers and timer management related stuff in the Linux kernel. In the previous part got acquainted with the `clockevents` framework. In this part we continied to learn time management related stuff in the Linux kernel and saw a little about three diferent clock sources which are used in the [x86](https://en.wikipedia.org/wiki/X86) architecture. The next part will be last part of this [chapter](https://0xax.gitbooks.io/linux-insides/content/Timers/index.html) and we will see some user space related stuff, i.e. how some time related [system calls](https://en.wikipedia.org/wiki/System_call) implemented in the Linux kernel.
This is the end of the sixth part of the [chapter](https://0xax.gitbooks.io/linux-insides/content/Timers/index.html) that describes timers and timer management related stuff in the Linux kernel. In the previous part got acquainted with the `clockevents` framework. In this part we continued to learn time management related stuff in the Linux kernel and saw a little about three diferent clock sources which are used in the [x86](https://en.wikipedia.org/wiki/X86) architecture. The next part will be last part of this [chapter](https://0xax.gitbooks.io/linux-insides/content/Timers/index.html) and we will see some user space related stuff, i.e. how some time related [system calls](https://en.wikipedia.org/wiki/System_call) implemented in the Linux kernel.
If you have questions or suggestions, feel free to ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-insides/issues/new).
Lajos Gerecs
8 years ago