Re: Bizarre results from perf event API

[David Ahern <dsahern@gmail.com>]

On 2/20/15 4:26 PM, Benjamin Moody wrote:
> I'm trying to write a program that uses the Linux perf event API to
> track the CPU usage of an arbitrary task.  I'm following the
> documentation at <http://linux.die.net/man/2/perf_event_open>.
>
> My program (highly simplified version shown below) seems to work most
> of the time, and as far as I can tell, it should be measuring exactly
> the same thing as what 'perf stat -e instructions:uD' measures.
> However, when I try measuring the performance of a certain large
> proprietary application, I get *completely* different results from my
> program and from perf stat.  (The results from perf are somewhat
> believable; mine are not.)
>

Trying using a program with established results. e.g., This is from 
PeterZ years ago. On intel 'perf stat -e instructions a.out' should show 
~1 billion (a wee bit more, but clearly in the 1b range).

int main (void)
{
         int i;

         for (i = 0; i < 100000000; i++) {
                 asm("nop");
                 asm("nop");
                 asm("nop");
                 asm("nop");
                 asm("nop");
                 asm("nop");
                 asm("nop");
         }

         return 0;
}