How to read performance counters on i5, i7 CPUs

Question

Modern CPUs have quite a lot of performance counters - http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html how to read them? I'm interested in cache misses and branch mispredictions.

Answer 1

Looks like PAPI has very clean API and works just fine on Ubuntu 11.04. Once it's installed, following app will do what I wanted:

#include <stdio.h>
#include <stdlib.h>
#include <papi.h>

#define NUM_EVENTS 4

void matmul(const double *A, const double *B,
        double *C, int m, int n, int p)
{
    int i, j, k;
    for (i = 0; i < m; ++i)
        for (j = 0; j < p; ++j) {
            double sum = 0;
            for (k = 0; k < n; ++k)
                sum += A[i*n + k] * B[k*p + j];
            C[i*p + j] = sum;
        }
}

int main(int /* argc */, char ** /* argv[] */)
{
    const int size = 300;
    double a[size][size];
    double b[size][size];
    double c[size][size];

    int event[NUM_EVENTS] = {PAPI_TOT_INS, PAPI_TOT_CYC, PAPI_BR_MSP, PAPI_L1_DCM };
    long long values[NUM_EVENTS];

    /* Start counting events */
    if (PAPI_start_counters(event, NUM_EVENTS) != PAPI_OK) {
        fprintf(stderr, "PAPI_start_counters - FAILED\n");
        exit(1);
    }

    matmul((double *)a, (double *)b, (double *)c, size, size, size);

    /* Read the counters */
    if (PAPI_read_counters(values, NUM_EVENTS) != PAPI_OK) {
        fprintf(stderr, "PAPI_read_counters - FAILED\n");
        exit(1);
    }

    printf("Total instructions: %lld\n", values[0]);
    printf("Total cycles: %lld\n", values[1]);
    printf("Instr per cycle: %2.3f\n", (double)values[0] / (double) values[1]);
    printf("Branches mispredicted: %lld\n", values[2]);
    printf("L1 Cache misses: %lld\n", values[3]);

    /* Stop counting events */
    if (PAPI_stop_counters(values, NUM_EVENTS) != PAPI_OK) {
        fprintf(stderr, "PAPI_stoped_counters - FAILED\n");
        exit(1);
    }

    return 0;
}

Tested this on Intel Q6600, it supports up to 4 performance events. Your processor may support more or less.

Answer 2

What about perf? perf list hw cache shows 33 different events and the man page shows how to use raw performance counter descriptors.

Answer 3

I think there is a available library that can be used, called perfmon2, http://perfmon2.sourceforge.net/, and documentations are available at http://www.hpl.hp.com/research/linux/perfmon/perfmon.php4 and http://www.hpl.hp.com/techreports/2004/HPL-2004-200R1.html, I am recently digging this lib out, I would post example code as soon as I figure it out~

Answer 4

Performance counters are read with the RDPMC insn.

EDIT: To add a bit more info, reading performance counters is not very easy and it would take pages upon pages if we are to describe it here, besides it involves writes to Model Specific Registers, which require privileged instructions. I would instead advise to use ready profilers - oprofile or Intel VTune, which are built upon performance counters.