The default frequency is misleading

[antocuni]

TL;DR; version: vmprof should run at 1000Hz, but it runs at 250Hz.

By default, vmprof uses a DEFAULT_PERIOD = 0.00099, which is almost 1000Hz. However, on all machines on which I tried linux seems to deliver SIGPROF at max 250Hz, so you get many less samples than what you would expect. But, if you use real_time=True, then it uses SIGALRM which seems to support higher frequency.

I think this is related to the CONFIG_HZ_* kernel settings; on my machine (ubuntu 16.10, kernel 4.8.0) I have:

$ cat /boot/config-`uname -r` | grep HZ
CONFIG_NO_HZ_COMMON=y
# CONFIG_HZ_PERIODIC is not set
CONFIG_NO_HZ_IDLE=y
# CONFIG_NO_HZ_FULL is not set
CONFIG_NO_HZ=y
# CONFIG_HZ_100 is not set
CONFIG_HZ_250=y
# CONFIG_HZ_300 is not set
# CONFIG_HZ_1000 is not set
CONFIG_HZ=250
CONFIG_MACHZ_WDT=m

You can easily see it by running this example, stolen from here. On my machine, I get "4016 Hz or higher" with SIGALRM, and "250 Hz" with SIGPROF.

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>


#define USECREQ 250
#define LOOPS   1000

void event_handler (int signum)
{
    static unsigned long cnt = 0;
    static struct timeval tsFirst;
    if (cnt == 0) {
        gettimeofday (&tsFirst, 0);
    } 
    cnt ++;
    if (cnt >= LOOPS) {
        struct timeval tsNow;
        struct timeval diff;
        setitimer (ITIMER_REAL, NULL, NULL);
        gettimeofday (&tsNow, 0);
        timersub(&tsNow, &tsFirst, &diff);
        unsigned long long udiff = (diff.tv_sec * 1000000) + diff.tv_usec;
        double delta = (double)(udiff/cnt)/1000000;
        int hz = (unsigned)(1.0/delta);
        printf ("kernel timer interrupt frequency is approx. %d Hz", hz);
        if (hz >= (int) (1.0/((double)(USECREQ)/1000000))) {
            printf (" or higher");
        }       
        printf ("\n");
        exit (0);
    }
}

int main (int argc, char **argv)
{
    struct sigaction sa;
    struct itimerval timer;

    memset (&sa, 0, sizeof (sa));
    sa.sa_handler = &event_handler;    
    //sigaction (SIGALRM, &sa, NULL);
    sigaction (SIGPROF, &sa, NULL);
    
    timer.it_value.tv_sec = 0;
    timer.it_value.tv_usec = USECREQ;
    timer.it_interval.tv_sec = 0;
    timer.it_interval.tv_usec = USECREQ;
    
    //setitimer (ITIMER_REAL, &timer, NULL);
    setitimer (ITIMER_PROF, &timer, NULL);
    while (1);
}