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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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

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