[ale] uptime weirdness

[kaboom at gatech.edu (Chris Ricker)]

On Thu, 28 Aug 2003 hbbs at comcast.net wrote:

> Jim -
> 
> I started playing around with powers of two to see if any of them came close to
> the number of seconds in 500 days.
> 
> 2^26 is closest, but not close enough to suggest there was a rollover.

jiffies are an unsigned long (see kernel/timers.c) and are incremented every 
1/100th of a second on Intel (normally)

2^32 / 86400 / 100 = 497.1027

As a result, jiffies wrap after 497.1 or so days on 32-bit hardware. Since 
uptime was calculated from jiffies, uptime also wrapped.

That's not a serious problem, but drivers sometimes use jiffies as a 
counter, and those get weird. They're supposed to have all been audited, 
but....

later,
chris
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