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The default frequency is misleading #163
Description
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);
}
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