RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

Dan Magenheimer
2009-11-04T21:29:36Z

> From: john stultz [mailto:johnstul@us.ibm.com]
> On Thu, Oct 29, 2009 at 7:07 AM, Avi Kivity <avi@redhat.com> wrote:
> >
> > Out of interest, do you know (and can you relate) why those
> apps need
> > 100k/sec monotonically increasing timestamps?
>
> This is sort of tangential, but depending on the need, this might be
> of interest: Recently I've added a new clock_id,
> CLOCK_MONOTONIC_COARSE (as well as CLOCK_REALTIME_COARSE), which
> return a HZ granular timestamp (same granularity as filesystem
> timestamps). Its very fast to access, since there's no hardware to
> touch, and is accessible via vsyscall.
>
> The idea being, if your hitting clock_gettime 100k/sec but you really
> don't have the need for nsec granular timestamps, it might provide a
> really nice performance boost.
>
> Here's the commit:

Hi John --

Yes, possibly of interest. But does it work with CONFIG_NO_HZ?
(I'm expecting that over time NO_HZ will become widespread
for VM OS's, though interested in if you agree.)

Also very interested in your thoughts about a variation
that returns something similar to a TSC_AUX to notify
caller that the underlying reference clock has/may have
changed.

Dan