performance counter 20% error finding retired instruction count

[Vince Weaver]

Hello

As an aside, is it time to set up a dedicated Performance Counters
for Linux mailing list?   (Hereafter referred to as p10c7l to avoid
confusion with the other implementations that have already taken
all the good abbreviated forms of the concept).  If/when the 
infrastructure appears in a released kernel, there's going to be a lot of 
chatter by people who use performance counters and suddenly find they are 
stuck with a huge step backwards in functionality.  And asking Fortran 
programmers to provide kernel patches probably won't be a productive 
response.  But I digress.


I was trying to get an exact retired instruction count from p10c7l.
I am using the test million.s, available here
 ( http://www.csl.cornell.edu/~vince/projects/perf_counter/million.s )
It should count exactly one million instructions.

Tests with valgrind and qemu show that it does.

Using perfmon2 on Pentium Pro, PII, PIII, P4, Athlon32, and Phenom
all give the proper result:

tobler:~% pfmon -e retired_instructions ./million
1000002 RETIRED_INSTRUCTIONS

   ( it is 1,000,002 +/-2 because on most x86 architectures retired
     instruction count includes any hardware interrupts that might
     happen at the time.  It woud be a great feature if p10c7l
     could add some way of gathering the per-process hardware
     instruction count statistic to help quantify that).

Yet with perf on the same Athlon32 machine (using
kernel 2.6.30-03984-g45e3e19) gives:

tobler:~%perf stat ./million

 Performance counter stats for './million':

       1.519366  task-clock-ticks     #       0.835 CPU utilization factor
              3  context-switches     #       0.002 M/sec
              0  CPU-migrations       #       0.000 M/sec
             53  page-faults          #       0.035 M/sec
        2483822  cycles               #    1634.775 M/sec
        1240849  instructions         #     816.689 M/sec # 0.500 per cycle
         612685  cache-references     #     403.250 M/sec
           3564  cache-misses         #       2.346 M/sec

 Wall-clock time elapsed:     1.819226 msecs

Running multiple times gives:
   1240849
   1257312
   1242313

Which is a varying error of at least 20% which isn't even 
consistent.  Is this because of sampling?  The documentation doesn't 
really warn about this as far as I can tell.

Thanks for any help resolving this problem

Vince


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