Vince,
Good points.
Also something else I am wondering about is: hardware page walker.
Does that influence instruction_retired somehow?
On Thu, Mar 18, 2010 at 4:11 PM, Vince Weaver <vweav...@eecs.utk.edu> wrote:
>
>> I wrote a simple Fibonacci and counted the #of instructions (inst_retired)
>> using both pin and performance counter.
>> As you can see, it seems like perf_counter undercount the #of instructions
>> and the result is non-deterministic (sometimes 94730 but sometimes 94729)
>> Any reason for this?
>
> Did you dynamically link your code? The dynamic linker under Linux does a
> lot of stuff which might not be deterministic.
>
> I compiled your code on an AMD Phenom machine.
>
> Dynamically linked, compiled with gcc 4.4 and -O2 it varied from
> 86558 to 86561.
>
> But if I statically linked, that is compile with "-static" I got
> a very consistent "7749" instructions each time.
>
> If you happen to have valgrind installed, version 3.5 or later, you can
> also count instructions using something like:
> valgrind --tool=exp-bbv --instr-count-only=yes -- ./fib
> although on your fib code the results are too high by a factor of two...
> weird. I need to find out why that happens.
>
> One thing to watch out for when using pin... newer versions of Pin (my
> notes say pinkit more recent than 29972) change the way that rep-prefixed
> string instructions are counted. Before that Pin matches what hardware
> does (each rep-prefixed instruction counts as "1") wheras current Pin
> counts each rep as a separate instruction. It is quite possible the
> overcount you see with Pin is due to this; the valgrind tool I mention
> above will tell you how many rep-prefixed instructions were executed in
> the code.
>
>
> One note on your test code... you should probably use the result of the
> "fib" calculation, as a printf or such. Otherwise the C compiler could
> optimize out the whole routine, as the result is unused.
>
>> main()
>> {
>> fib(10)
>> }
>>
>>
>>
>>
>
> Vince
> vweav...@eecs.utk.edu
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