Re: [PATCH 2/3] High-res-timers part 2 (x86 platform code) take 5.1

From: Vojtech Pavlik (
Date: Mon Oct 14 2002 - 02:18:56 EST

On Sun, Oct 13, 2002 at 12:46:31PM +0200, Ingo Adlung wrote:

> Linus Torvalds wrote:
> > On Wed, 9 Oct 2002, george anzinger wrote:
> >
> >>This patch, in conjunction with the "core" high-res-timers
> >>patch implements high resolution timers on the i386
> >>platforms.
> >
> >
> > I really don't get the notion of partial ticks, and quite frankly, this
> > isn't going into my tree until some major distribution kicks me in the
> > head and explains to me why the hell we have partial ticks instead of just
> > making the ticks shorter.

Not speaking for a major distro, just for me writing HPET (high
performance event timer ...) support for x86-64 (and it happens to exist
on ia64 as well, and possibly might be in new Intel P4 chipsets, too).

It's a very nice piece of hardware that allows very fine granularity
aperiodic interrupts (in each interrupt you set when the next one will
happen), without much overhead.

It'd be a shame to just set this timer to 1kHz periodic just use that as
a base timer, when you can do much better resolution and latency-wise.
HPET has a base clock > 10 MHz.

> > Linus
> In any kind of virtual environment you would rather prefer a completely
> tickless system alltogether than increased tick rates. In a S/390
> virtual machine, running many hundreds of virtual Linux servers the
> 100Hz timer pops are already considerably painful, and going to a higher
> tick rate achieving higher timer resolution is completely prohibitive.
> Similar is true in many embedded systems related to power consumption of
> high frequency ticks.
> However, George has shown that introducing the notion of a completely
> tickless system is expensive on Intel overhead wise, thus partial ticks
> seem to be a possibility addressing the needs for embedded and virtual
> environments, getting decent timer resolution as needed.

When HPET becomes a standard (yes, it's a MS requirement for new PCs),
it won't be expensive on i386 anymore.

Vojtech Pavlik
SuSE Labs
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