On Thu, 14 Jul 2005, Linus Torvalds wrote:
>
> But what you can do is to have HZ at some reasonably high value (ie in the
> kHz range), and then slow down the system clock to conserve energy, and
> increment jiffies by 16 or 32 when in "slow clock mode".
Btw, it doesn't have to even be a slow-down due to the kernels decision.
In a VM environment, the timer interrupt might be erratic, and the timer
interrupt might read some hardware register (TSC or something) and use
_that_ to increment jiffies by the "proper" amount.
See? The point is that "jiffies" is useful exactly because it's very cheap
to read portably (there are no portable high-performance alternatives) and
because it has the right resolution to be useful in 32 bits.
That doesn't mean that the code that updates it can't be clever. We
already have code that updates it that is a lot more intelligent than 99%
of the code that reads it: we update it in 64 bits under xtime_lock, even
though most readers use a lock-less 32-bit read and only get a partial
value - the part they care about.
And this is a wonderful property that everybody seems to be ignoring, even
though we have absolutely tons of code that just takes all of this
goodness for granted.
Linus
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