Question

How do I manually convert jiffies to milliseconds and vice versa in Linux? I know kernel 2.6 has a function for this, but I’m working on 2.4 (homework) and though I looked at the code it uses lots of macro constants which I have no idea if they’re defined in 2.4.

Answer 1

As a previous answer said, the rate at which jiffies increments is fixed.

The standard way of specifying time for a function that accepts jiffies is using the constant HZ.

That’s the abbreviation for Hertz, or the number of ticks per second. On a system with a timer tick set to 1ms, HZ=1000. Some distributions or architectures may use another number (100 used to be common).

The standard way of specifying a jiffies count for a function is using HZ, like this:

schedule_timeout(HZ / 10);  /* Timeout after 1/10 second */

In most simple cases, this works fine.

2*HZ     /* 2 seconds in jiffies */
HZ       /* 1 second in jiffies */
foo * HZ /* foo seconds in jiffies */
HZ/10    /* 100 milliseconds in jiffies */
HZ/100   /* 10 milliseconds in jiffies */
bar*HZ/1000 /* bar milliseconds in jiffies */

Those last two have a bit of a problem, however, as on a system with a 10 ms timer tick, HZ/100 is 1, and the precision starts to suffer. You may get a delay anywhere between 0.0001 and 1.999 timer ticks (0-2 ms, essentially). If you tried to use HZ/200 on a 10ms tick system, the integer division gives you 0 jiffies!

So the rule of thumb is, be very careful using HZ for tiny values (those approaching 1 jiffie).

To convert the other way, you would use:

jiffies / HZ          /* jiffies to seconds */
jiffies * 1000 / HZ   /* jiffies to milliseconds */

You shouldn’t expect anything better than millisecond precision.