Weird timing results in C Post: 302351183

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Top Forums Programming Weird timing results in C Post 302351183 by CRGreathouse on Monday 7th of September 2009 12:16:02 PM

09-07-2009

Registered User

Weird timing results in C

I was running some timings in my code to see which of several functions was the best and I've been getting some odd results. Here's the code I'm using:

Code:

static double time_loop(int (*foo)(int)) {
	clock_t start, end;
	int n = 0, i = 0;
	start = clock();
	for (; i <= MAXN; i++)
		if ((*foo)(i)) n++;
	end = clock();
	if (n != 50847534)
		return -1;	// Error
	return ((double) (end - start)) / CLOCKS_PER_SEC;
}

static double median3(int (*foo)(int)) {
	double t1 = time_loop(foo);
	double t2 = time_loop(foo);
	double t3 = time_loop(foo);
	printf("  ((%f %f %f))\n", t1, t2, t3);
	int errors = 0;
	if (t1 < 0)
		errors++;
	if (t2 < 0)
		errors++;
	if (t3 < 0)
		errors++;
	if (errors > 0)
		return -errors;
	if (t1 > t2) {
		if (t2 > t3)
			return t2;
		return t3 < t1 ? t3 : t1;
	} else {
		if (t1 > t3)
			return t1;
		return t3 < t2 ? t3 : t2;
	}
}

The functions that I'm passing to time_loop are pure functions -- they're not making any changes to global variables. And yet every time I run this, my debugging line in median3 shows that the third invocation takes significantly longer than the others. Sample run:

Code:

  ((4.560000 3.750000 6.540000))
Original: 4.560000 s
  ((4.920000 3.760000 6.690000))
Original: 4.920000 s
  ((4.190000 3.750000 6.620000))
Original: 4.190000 s
  ((3.820000 3.130000 5.640000))
New:      3.820000 s
  ((3.580000 3.150000 5.580000))
New:      3.580000 s
	  ((3.670000 3.140000 5.510000))
New:      3.670000 s

Why is that? These functions are entirely deterministic, and nothing is changing between invocations. But I see a strong pattern of time - smaller time - much longer time.

As for cache effects, the functions rely on a 125 MB global array, so that's not going to fit even in my 6MB L3. (Even if it did you'd expect the second and third invocations to be fast.)

CRGreathouse

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LEARN ABOUT CENTOS

clock

CLOCK(3)						     Linux Programmer's Manual							  CLOCK(3)

NAME

       clock - determine processor time

SYNOPSIS

       #include <time.h>

       clock_t clock(void);

DESCRIPTION

       The clock() function returns an approximation of processor time used by the program.

RETURN VALUE

       The value returned is the CPU time used so far as a clock_t; to get the number of seconds used, divide by CLOCKS_PER_SEC.  If the processor
       time used is not available or its value cannot be represented, the function returns the value (clock_t) -1.

CONFORMING TO

       C89, C99, POSIX.1-2001.	POSIX requires that CLOCKS_PER_SEC equals 1000000 independent of the actual resolution.

NOTES

       The C standard allows for arbitrary values at the start of the program; subtract the value returned from a call to clock() at the start	of
       the program to get maximum portability.

       Note  that  the	time  can  wrap  around.   On a 32-bit system where CLOCKS_PER_SEC equals 1000000 this function will return the same value
       approximately every 72 minutes.

       On several other implementations, the value returned by clock() also includes the times of any children whose status has been collected via
       wait(2)	(or  another  wait-type  call).   Linux  does  not include the times of waited-for children in the value returned by clock().  The
       times(2) function, which explicitly returns (separate) information about the caller and its children, may be preferable.

SEE ALSO

       clock_gettime(2), getrusage(2), times(2)

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2008-08-28								  CLOCK(3)

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