RE: [Xen-devel] [RFC] Correct/fast timestamping in apps under Xen [1 of 4]: Reliable TSC

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> On 10/09/09 14:35, Dan Magenheimer wrote:
> > Excellent!  This is an extremely important piece
> > of the puzzle now filled in.
> >
> > Just for completeness, on your machine, what is
> > the measurement for raw rdtsc?
> >   
> 
> A naked inline rdtsc is about 30ns, so only about a factor of 
> 3 better. 
> Which is a surprisingly small improvement given that the full
> gettimeofday path has ~150 instructions, including a couple of
> multiplies, quite a few jumps and two "lsl" instructions for vgetcpu
> (which each cost about 10ns).  rdtsc is an expensive instruction...
> 
>     J

Very nice!

One more measurement if you haven't already torn down
your test environment:   If you are at xen-unstable tip,
with tsc emulation on, please try something like:

for i in {0..100}; do
xm debug-key s; xm dmesg | tail; sleep 1;
done

to get an idea of the number of rdtsc's being
done per second (and also divide by the number
of cores so we have rdtsc's/sec/core).  This is
of course unloaded, so if you have a favorite
load to throw on it, that would be very interesting
also.

(Note that the s debug-key may be slow because
xen is also now running check_tsc_warp each time.)

Thanks,
Dan

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