convert jiffies to seconds

Question

I've got a piece of userspace code which is parsing /proc/PID/task/TID/stat to get the cpu usage. I can use HZ to get the jiffies per second but this code could move to another machine which has a different configured value. Is there any way to get the value of HZ from userspace at runtime?

Duplicate: http://stackoverflow.com/questions/2731463/converting-jiffies-to-milli-seconds — Daenyth, Oct 06 '10 at 19:34

score 7 · Accepted Answer · edited Mar 03 '23 at 15:20

7

You divide it by the number you get from sysconf(_SC_CLK_TCK).

However, I think this is probably always 100 under Linux regardless of the actual clock tick, it's always presented to userspace as 100.

See man proc 5.

edited Mar 03 '23 at 15:20

Błażej Michalik

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answered Oct 06 '10 at 22:09

MarkR

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I did read that 100 is always presented to userspace... but wasn't sure /proc counted as userspace :) – Brian Makin Oct 06 '10 at 22:43
I'm not certain I understand: does this mean that HZ is not actually the number that the questioner wanted? – Eric Seppanen Oct 07 '10 at 15:40
1

HZ is the number I wanted... but it is a compiled in constant. As long as I'm running on the same machine I compiled for... all is good. But if I move to another machine without recompiling then it might have the wrong value. – Brian Makin Oct 08 '10 at 13:55

score 4 · Answer 2 · edited May 23 '17 at 12:22

To clarify the math behind MarkR's answer:

sysconf(_SC_CLK_TCK) will get you jiffies per second. Divide jiffies by the number you get from sysconf(_SC_CLK_TCK) to get the total number of seconds.

      jiffies                      jiffies              seconds
--------------------    =     -----------------    =    -------    =    seconds
sysconf(_SC_CLK_TCK)          (jiffies/second)             1

score 0 · Answer 3 · answered Aug 26 '16 at 09:08

For shell-scripting, etc, use getconf CLK_TCK from the command-line. Use can use this to pass that parameter in as an environment variable or on the command-line.

main(int argc, char **argv) { 
    unsigned long clk_tck = atol(
        getenv("CLK_TCK") || "0"
    ) || sysconf(_SC_CLK_TCK) ;
    ... /* your code */

This uses the sysconf as above, but allows you to override it with an environment variable, which can be set with the above command.

score 0 · Answer 4 · answered Jul 22 '11 at 07:09

Source of "ps" command include file <linux/param.h> to get value of HZ.

They also look for an "ELF note" with number 17 to find value of HZ (sysinfo.c):

 //extern char** environ;

 /* for ELF executables, notes are pushed before environment and args */
 static unsigned long find_elf_note(unsigned long findme){
   unsigned long *ep = (unsigned long *)environ;
   while(*ep++);
   while(*ep){
     if(ep[0]==findme) return ep[1];
     ep+=2;
   }
   return NOTE_NOT_FOUND;
 }
 [...]
 hz = find_elf_note(17);

I have to admit it look weird for me since ELF notes is a section defined during compilation.

convert jiffies to seconds

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