[Click] click performance test

[Eddie Kohler]

Hi Jiangbo,

> I am doing performance test on click elements. I use RDTSC 
> to read the performance counter.  In your thesis, you got the 
> very accurate results like checkipheader takes 457ns/packet.
> How can you get the result so accurate? 

As I remember, we report the mean of several trials. We didn't report
standard deviation numbers, so I couldn't tell you the error range. As the
thesis says,

Each element's cost is the difference between the Pentium III cycle counter
value before and after executing the element, decreased by the time
required to read the cycle counter; it includes the virtual function
call(s) that move a packet from one element to the next.

> Another problem is when I do two continuous RDTSC, It took 85
> cycles. Seems to much. Is there anything I missed?

Probably not; rdtsc is quite expensive; but I'm not positive about
particular numbers.

Eddie