Re: Accessing monotonic clock from modules

[Lee Revell]

> Arjan van de Ven wrote:
> > On Thu, 2005-06-02 at 07:46 -0600, Chris Friesen wrote:
>
> > > For ourselves we implemented an clock interface for a limited subset of 
> > > architectures that provides a fast timestamp in kernel and userspace.
> > >
> > > Basically it has one call to return a 64-bit timestamp, and another call 
> > > to tell you how fast the clock is ticking.
> >
> >
> > hmm this is tricky if cpufreq actually varies cpu speeds... you would
> > need to not cache the "how fast it ticks" for too long.
>
> Luckily we didn't need to deal with that.
>
> In order to use the fast versions with varying frequency you'd need some 
> kind of notification to all users when the frequency changes.
>
> Alternately, on architectures where clock_gettime doesn't require the 
> overhead of a syscall, you could just use that.

JACK also implements a fast high-res timer for each supported arch.  We
can't use gettimeofday as it's about 50x slower than rdtsc (tested it just 
now).

I suggested using the kernel events mechanism to notify userspace when
the CPU speed changes a few months ago, for this exact reason.

Lee

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/