# What is a CPU tick?

### Question:

• How is a CPU tick calculated and what does it represent?
• Does a single `tick` equate to `10 miliseconds` thus if some thread reported not called for (`5 * 10 ticks = 500 ticks`) does this mean the CPU was perhaps too busy to schedule the aforementioned thread to work?
• could you put "CPU tick" in context - perhaps cut and paste the paragraph of the source of the phrase. I am concerned that there could be confusion between one of several possible answers.
– Mick
Jan 28, 2010 at 9:44
• A common value is 16.666 ms (from the 60 Hz tick counter, going back to the original IBM PC). Jan 24 at 21:56

A tick is an arbitrary unit for measuring internal system time. There is usually an OS-internal counter for ticks; the current time and date used by various functions of the OS are derived from that counter.

How many milliseconds a tick represents depends on the OS, and may even vary between installations. Use the OS's mechanisms to convert ticks into seconds.

As to why a thread reports it's not being called: That will depend on whether the thread is blocking somewhere (waiting, I/O etc.). If it is not blocking, then yes, the OS's scheduler will decide when it gets to run, which may be a long time if the system is busy.

Edit:

Note that, perhaps unfortunately, some authors also use tick as a synonym for processor clock cycle (e.g. this text). I believe this usage is less widespread, but still, best to find out first what people are talking about.

• So the CPU requires a fixed number of clock ticks to execute each instruction? Jan 27, 2010 at 13:12
• @aaron: no, instruction execution time is bound to a certain number of processor cycles. given a specific OS on a specific CPU running at a specific frequency, you can calculate how many ticks a specific instruction takes to execute, but that calculation isn't necessarily valid for any other combination of OS/CPU/frequency/instruction. Jan 27, 2010 at 13:41
• @Aaron: No, ticks and processor cycles are two different concepts. Processor cycle lenght is determined by the hardware (CPU frequency); ticks are produced by the OS and use whatever length the OS (or its designers) deem appropriate. Jan 28, 2010 at 9:03
• @sleske Hmmm I see the difference. 'ticks' used to be a synonym for CPU cycles, as well as a term for "a constant amount of time independent of CPU clock speed". Same term with two meanings. Is that right? Jan 28, 2010 at 9:14
• @quackquixote That is not even true anymore. In a pipeline architecture, I can do multiple things in a CPU cycle sometimes, and in other times I can do only one, perhaps because the pipeline was filled with useless predictions that are wasted because the code took an unexpected direction, etc. You would have a hard time correlating CPU cycles to real time even for the same system running different code. Jan 16, 2015 at 17:39

Edit: Taken from PC Hardware in a Nutshell:

"The processor clock coordinates all CPU and memory operations by periodically generating a time reference signal called a clock cycle or tick. Clock frequency is specified in gigahertz (GHz), which specifies billions of ticks per second. Clock speed determines how fast instructions execute. Some instructions require one tick, others multiple ticks, and some processors execute multiple instructions during one tick."

The time between ticks is determined by your clock speed, and it takes one to many ticks depending on the OP being performed. For example, a 286 class CPU needs 20 ticks to multiply two numbers.

If you need high performance timers, then I don't think you can rely on ticks being constant across all systems.

The CPU scheduler could have delayed the thread, especially if there was another thread with a higher priority. So yes, the CPU could've been too busy.

• -1 You are confusing ticks and processor cycles. `ticks` on a Unix system usually occur 60 or 100 times per second, and are not bound to proc speed. Jan 27, 2010 at 12:33
• I'm talking about hardware ticks. a 1hz CPU processes 1 tick (cycle) per second, 200Hz can process 200 cycles per second, 2GHz two billion cycles/sec. The faster your CPU, the more cycles/sec you get. Jan 28, 2010 at 8:54
• Ah, I see, some authors use `tick` as a synonym for `processor cycle`. Still, I mostly see `tick` used as explained in my answer above. But I guess terminology varies, as usual :-(. I edited my answer. Jan 28, 2010 at 9:08

In addition to sleske's answer I wrote a C program that will print the "OS ticks per second" `ft`:

``````// C headers
#include <stdio.h>

#include <unistd.h>

int main(int argc, char * argv[]){

long ft; // OS ticks per second

t = sysconf(_SC_CLK_TCK);
printf("ft = %ld\n\n", ft);

return 0;
}
``````

In my case I am on OS Linux Debian system and when I compile this C program and run it, it will always print `ft = 100`. Note that here unit is not written but it is "ticks per second". Therefore this is a frequency! We can write it with a unit:

`ft = 100 1/s`

If we want to get a period `t` out of frequency `ft` this equation is the missing link.

`t = 1/ft``t = 1/(100 1/s)``t = 1/100 s`

Let's convert this in ˙ms˙:

`t = 1/100 s = (1 * 10^(-3)) s / (100 * 10^(-3)) = 1 * m s / (100 * 10^(-3)) = 1 * m s * 10^3 * 10^(-2) = 10^4 ms = 10 ms`.

So tick happens every `10 ms` on OS Linux Debian.