perf pebs sampling through stores + period is wrong?

[Harald Servat]

Dear all,

  I'd let you know that I'm observing that the PEBS sampling through 
PEBS stores seem to behave badly (at least to my understanding) in 
cooperation with -c flag.

  I'm running Linux 3.11.0 on a Intel SandyBridge machine with the 
following info

vendor_id       : GenuineIntel
cpu family      : 6
model           : 42
model name      : Intel(R) Core(TM) i7-2760QM CPU @ 2.40GHz
stepping        : 7

  and for testing purposes I'm using the attached program (which simply 
transfers data from one vector to another one) to depict the problem. 
When I use perf stat to get information of the loads & stores of this 
app I get this output (which I reduced manually)

 $ perf stat -e r81d0 ./a.out  # Intel manual [1] in table 19-17 
indicates that event number d0 + umask 81 refers to all loads
  671.488.050 loads

 $ perf stat -e r82d0 ./a.out # The same as before, but for stores
  356.521.360 stores

  We can see there that the number of stores is half the number of 
loads. However, when I use the perf mem record command for every 10k 
loads I get the following info:

  $ perf mem -t load record -c 10000 ./a.out
  [perf record: Woken up 1 times to write data]
  [perf record: Captured and wrote 0.047 MB perf.data (~2036 samples)]

  but when looking for samples every 10k stores I get

  $ perf mem -t store record -c 10000 ./a.out
  ...
  [perf record: Woken up 4 times to write data]
  [perf record: Captured and wrote 0.921 MB perf.data (~40247 samples)]

  Notice that the number of samples raised by 20x, which to me seems 
very odd because the number of stores was half, so I expected 0.5x here. 
Or am I supposing this the wrong way?

  Just for further testing, if I omit the -c parameter (which I need 
:S), it seems to work better

  $ perf mem -t load record  ./a.out
  [perf record: Woken up 1 times to write data]
  [perf record: Captured and wrote 0.172 MB perf.data (~7508 samples)]

  $ perf mem -t store record  ./a.out
  [perf record: Woken up 1 times to write data]
  [perf record: Captured and wrote 0.151 MB perf.data (~6607 samples)]

Best regards.

[1] 
http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-vol-3b-part-2-manual.pdf


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http://www.bsc.es/disclaimer#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *long_str = "This is a very long string!";
char dest[1024*1024*1024];

int main (int argc, char *argv[])
{
	int i;
	int length = strlen (long_str);

	for (i = 0; i < 1024*1024*1024-length; i += length)
		memcpy (&dest[i], long_str, length);

	printf ("CHECK: %c\n", dest[0*length+0]);
	printf ("CHECK: %c\n", dest[1*length+1]);
	printf ("CHECK: %c\n", dest[2*length+2]);
	printf ("CHECK: %c\n", dest[3*length+3]);

	return 0;
}