Question

I am trying to create a timer to profile some functions. For example,

#include <date_time/posix_time/posix_time.hpp>

boost::posix_time::ptime t1, t2;
t1 = boost::posix_time::microsec_clock::local_time();
boost::this_thread::sleep(boost::posix_time::milliseconds(1000));
t2 = boost::posix_time::microsec_clock::local_time();

std::cout << to_simple_string(t1) << std::endl;
std::cout << to_simple_string(t2) << std::endl;

The output I get is this,

2012-Mar-04 08:21:24.760019
2012-Mar-04 08:21:25.760577

The actual time elapsed is 1 second and 558 micro second, is there anyway that I can improve this accuracy to say under 1 microsecond?

I also tried something like this, with linker -lrt.

#include <time.h>
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &timer_start);
boost::this_thread::sleep(boost::posix_time::milliseconds(1000));
clock_gettime(1CLOCK_PROCESS_CPUTIME_ID, &timer_end);

But the compiler returns a semantic error for CLOCK_PROCESS_CPUTIME_ID. The error message is as follow.

Description Resource    Path    Location    Type
Symbol 'CLOCK_PROCESS_CPUTIME_ID' could not be resolved _OVERVIEW.cpp   /FUTETP/BACKEND line 32 Semantic Error

Answer 1

Please keep in mind two things:

1. sleep() and other time-sensitive commands could be inaccurate for several reasons (system load, etc.). You will never get within 1 microsecond accuracy. 500 microseconds isn’t too bad for timing sleep()
2. If your system time is controlled by NTP, etc. and the clock is changed during your timing operation, you will see this. If the time is rolled back, it is possible to obtain a negative (or large unsigned number).

By the way, I’m unsure of what method Boost is using to get the time, but consider gettimeofday(2) from sys/time.h (or newer clock_gettime() from time.h). It returns the time since midnight Jan 1, 1970. Record the time before and after and subtract. Here’s an example of gettimeofday().

#include <sys/time.h>

#include <stdio.h>
#include <unistd.h>

unsigned int
timeDif(
    struct timeval *before,
    struct timeval *after)
{
    return ((after->tv_sec * 1000000 + after->tv_usec) - 
        (before->tv_sec * 1000000 + before->tv_usec));
}

int
main(
    int argc,
    char *argv[]) {
    struct timeval before, after;

    gettimeofday(&before, NULL);
    sleep(1);
    gettimeofday(&after, NULL);

    printf("Elapsed time: %u microseconds\n", timeDif(&before, &after));

    return (0);
}