Weird timing results in C Post: 302351183

Sponsored Content

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

View Public Profile for CRGreathouse

Find all posts by CRGreathouse

10 More Discussions You Might Find Interesting

1. UNIX for Advanced & Expert Users

scp timing out

having problems using scp in that during peak hours it appears to time out. anyone have similar experiences? any thoughts regarding a solution...

2. Shell Programming and Scripting

Timing out a SSH

I need to make it so an autmated process which involves ssh, times out if ssh prompts for a password. Most of the time it shouldnt prompt for a password. But if it does i need it to time it out or get a status and stop the ssh and log that the ssh failed and move onto the next server. Is there any...

3. Programming

timing your functions

hi everyone. If you have a function created in your code and you want to find out how long it takes for it to run you can use a struct called gettimeofday(). so lets say we have a function like this int myfunction (int r) { /*some math calculations*/ return answer; } How do i set up...

4. Programming

Help with __builtin_prefetch function and it's timing

Hello there, I just needed to know how to get the timing right when using the gcc __builtin_prefetch() function, that is, how many instructions before the actual utilization of the data should I make the prefetch call. I will be measuring the L1 cache hit rate with valgrind's cachegrind,...

5. UNIX for Advanced & Expert Users

byte swapping 32-bit float and weird od results

I'm attempting to read a file that is composed of complex 32-bit floating point values on Solaris 10 that came from a 64-bit Red Hat computer. When I first tried reading the file, it looked like there was a byte-swapping problem and after running the od command on the file Solaris and Red Hat...

6. Shell Programming and Scripting

Can ctag and cscope support recording search results and displaying the history results ?

Hello , When using vim, can ctag and cscope support recording search results and displaying the history results ? Once I jump to one tag, I can use :tnext to jump to next tag, but how can I display the preview search result?

7. Shell Programming and Scripting

Timing a script

i have a very big script i have that i'd like to add a timeout to. this script runs on a several remote host. i update this script with timeout clause and then copy it over to all the hosts on which it is currently on. basically, i want the timeout to make the script abort/exit if it's...

8. Shell Programming and Scripting

Timing the shell script

I have two shell scripts, one written with xargs for parallel processing (p1) and the other written in old school way (p3) . when I execute them, i get the below values. $ time ./p1 real 0m25.36s user 0m0.32s sys 0m0.80s $ time ./p3 real 0m23.25s user 0m6.20s sys ...

9. Red Hat

Hardware and system timing are different

-> We have 2 servers server1 and server2 server. ->server1 is master application and server2 is slave application server. ->output of server1 hardware and slave timing: # hwclock --show Thu 05 Jun 2014 05:34:08 PM SGT -0.465666 seconds # date Thu Jun 5 17:34:16 SGT 2014 # cd...

10. UNIX for Beginners Questions & Answers

Weird 'find' results

Hello and thanks in advance for any help anyone can offer me I'm trying to learn the find command and thought I was understanding it... Apparently I was wrong. I was doing compound searches and I started getting weird results with the -size test. I was trying to do a search on a 1G file owned by...

LEARN ABOUT DEBIAN

qwtsystemclock

QwtSystemClock(3)						 Qwt User's Guide						 QwtSystemClock(3)

NAME

       QwtSystemClock -

       QwtSystemClock provides high resolution clock time functions.

SYNOPSIS

       #include <qwt_system_clock.h>

   Public Member Functions
       QwtSystemClock ()
       virtual ~QwtSystemClock ()
       bool isNull () const
       void start ()
       double restart ()
       double elapsed () const

   Static Public Member Functions
       static double precision ()

Detailed Description
       QwtSystemClock provides high resolution clock time functions.

       Sometimes the resolution offered by QTime ( millisecond ) is not accurate enough for implementing time measurements ( f.e. sampling ).
       QwtSystemClock offers a subset of the QTime functionality using higher resolution timers ( if possible ).

       Precision and time intervals are multiples of milliseconds (ms).

       Note:
	   The implementation uses high-resolution performance counter on Windows, mach_absolute_time() on the Mac or POSIX timers on other
	   systems. If none is available it falls back on QTimer.

Constructor &; Destructor Documentation
   QwtSystemClock::QwtSystemClock ()
       Constructs a null clock object.

   QwtSystemClock::~QwtSystemClock () [virtual]
       Destructor.

Member Function Documentation
   double QwtSystemClock::elapsed () const Returns:
	   Number of milliseconds that have elapsed since the last time start() or restart() was called or 0.0 for null clocks.

   bool QwtSystemClock::isNull () const Returns:
	   true if the clock has never been started.

   double QwtSystemClock::precision () [static] Returns:
	   Accuracy of the system clock in milliseconds.

   double QwtSystemClock::restart () The start time to the current time and return the time, that is elapsed since the previous start time.
   void QwtSystemClock::start () Sets the start time to the current time.
Author
       Generated automatically by Doxygen for Qwt User's Guide from the source code.

Version 6.0.0							  Fri Apr 15 2011						 QwtSystemClock(3)

10 More Discussions You Might Find Interesting

1. UNIX for Advanced & Expert Users

scp timing out

Discussion started by: jph

2. Shell Programming and Scripting

Timing out a SSH

Discussion started by: rcunn87

3. Programming

timing your functions

Discussion started by: bebop1111116

4. Programming

Help with __builtin_prefetch function and it's timing

Discussion started by: Tavo