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Re: Jiffies

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On Fri, 16 May 2003 10:45:12 +0200
"Joeri Belis" <joeri.belis@nollekens.be> wrote:

>I looked at our timer and it is at  : 1577504985
>
>And if  i understand correctly, this is  a 32bit value and  it can go
>to: 4294967295.  What will happen when we hit this limit?

As you correctly said, jiffies is defined as unsigned long (which is a
32  bit value  on  x86 platforms)  and  it is  incremented  by one  by
do_timer() function every  tick. In particular, you have  HZ ticks per
second with HZ =  100 on Intel x86 - kernel 2.4.x.  This means you can
experience a  jiffy overflow. Kernel  code is aware of  this situation
and so it handles it properly.  If you write a device driver, probably
sometimes you  could be forced to  do some checks even  if it's rarely
adopted.   F.e. if  you are  measuring time  intervals you  could just
think about a solution in which you evaluate the sign of

	delta = time_2 - time_1

If it is lesser than 0, jiffies has wrapped around and so in this case
you can avoid problems doing this

	delta = (MAX_JIFFIES_VALUE - time_1) + time_2

thus obtaining something similar

	[..]
	delta = time_2 - time_1;
	if (delta < 0)
		delta += MAX_JIFFIES_VALUE;
	[..]


Regards.

- --

Angelo Dell'Aera 'buffer' 
Antifork Research, Inc.	  	http://buffer.antifork.org

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