Re: Semaphore assembly-code bug

[dean gaudet]

On Sun, 31 Oct 2004, linux-os wrote:

> Timer overhead = 88 CPU clocks
> push 3, pop 3 = 12 CPU clocks
> push 3, pop 2 = 12 CPU clocks
> push 3, pop 1 = 12 CPU clocks
> push 3, pop none using ADD = 8 CPU clocks
> push 3, pop none using LEA = 8 CPU clocks
> push 3, pop into same register = 12 CPU clocks

your microbenchmark makes assumptions about rdtsc which haven't been valid 
since the days of the 486.  rdtsc has serializing aspects and overhead 
that you can't just eliminate by running it in a tight loop and 
subtracting out that "overhead".

you have to run your inner loops at least a few thousand of times between 
rdtsc invocations and divide it out to find out the average cost in order 
to eliminate the problems associated with rdtsc.

-dean

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