Re: CPU time versus wall clock time
From: Alex Vinokur (alexvn_at_bigfoot.com)
Date: 10/30/03
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Date: Thu, 30 Oct 2003 07:10:09 +0200
"Scott Kelly" <scottjk@softhome.net> wrote in message news:80m0qvopu4cijkube7hv6tl385ep3h30uk@4ax.com...
[snip]
> Considering I can't read C++ (yet), and yes, I'm a novice at C, that
> didn't help. It just showed me that clock time and cpu time can
> differ.
[snip]
OK.
Here is a C related sample.
time() and clock() return different values.
* `time' -- get current calendar time (as single number) - from 'man time'.
Returned value is number of _seconds_.
Elapsed (_wall clock_) time is = end_current_calendar_time - start_current_calendar_time.
* `clock' -- cumulative processor time
Returned value is number of _units_ defined by the machine-dependent macro `CLOCKS_PER_SEC'.
_Processor_ time used is = end_clock_units - start_clock_units.
=======================================
GNU gcc version 3.3.1 (cygming special)
=======================================
========= C++ code : File foo.c : BEGIN =========
#include <stdio.h>
#include <time.h>
#include <assert.h>
#include <unistd.h> // UNIX
int main()
{
time_t start_time, end_time;
clock_t start_clock, end_clock;
clock_t diff_clock;
time_t diff_time;
start_time = time(NULL);
if (start_time == (time_t)(-1))
{
fprintf (stderr, "Unable to get start time()\n");
return 1;
}
start_clock = clock();
if (start_clock == (clock_t)(-1))
{
fprintf (stderr, "Unable to get start clock()\n");
return 1;
}
sleep (3); // do nothing 3 seconds; UNIX, system call
end_time = time(NULL);
if (end_time == (time_t)(-1))
{
fprintf (stderr, "Unable to get end time()\n");
return 1;
}
end_clock = clock();
if (end_clock == (clock_t)(-1))
{
fprintf (stderr, "Unable to get end clock()\n");
return 1;
}
// -------------------------
assert (!(start_time == (time_t)-1));
assert (!(end_time == (time_t)-1));
assert (!(start_clock == (clock_t)-1));
assert (!(end_clock == (clock_t)-1));
// -------------------------
diff_time = end_time - start_time;
diff_clock = end_clock - start_clock;
printf ("time : start_time = %lu sec, end_time = %lu sec\n", start_time, end_time);
printf ("time : elapsed (wall clock) time = %lu sec\n", diff_time);
printf ("\n");
printf ("clock : start_clock = %lu units, end_clock = %lu units\n", start_clock, end_clock);
printf ("clock : processor time used = %.3f sec\n", (double)diff_clock/CLOCKS_PER_SEC);
return 0;
}
========= C++ code : File foo.c : END ===========
========= Compilation & Run : BEGIN ===========
$ gcc -W -Wall foo.c
$ a
time : start_time = 1067489895 sec, end_time = 1067489898 sec
time : elapsed (wall clock) time = 3 sec
clock : start_clock = 10 units, end_clock = 30 units
clock : processor time used = 0.020 sec
========= Compilation & Run : END =============
We can see that
* elapsed (wall clock) time (in other words - real time) is 3 sec,
* processor time used is only 0.020 sec.
--
=====================================
Alex Vinokur
mailto:alexvn@connect.to
http://mathforum.org/library/view/10978.html
news://news.gmane.org/gmane.comp.lang.c++.perfometer
=====================================
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