Re: Inconsistent timing results of multithreaded program on an SMP machine.

[Paul Jackson <pj@sgi.com>]

Marcel wrote:
> Instead what I've got was an oscillation where the maximum time was twice
> and more than the minimum!! For a short test results ranged ~7sec to ~16 ...

Just for grins, try displaying which cpu each thread runs on.  Display the
return from "latestcpu(getpid())" in the two per-thread printf's, to display
the thread's cpu at the beginning and end of each compute_thread().  Perhaps
you will notice that the per-thread cpu correlates with the test times.

/*
 * int latestcpu(pid_t pid)
 *
 * Copyright (C) 2005 Silicon Graphics, Inc.
 * This code is subject to the terms and conditions of the
 * GNU General Public License.
 *
 * Return most recent CPU on which task pid executed.
 *
 * The last used CPU is visible for a given pid as field #39
 * (starting with #1) in the file /proc/<pid>/stat.  Currently
 * this file has 41 fields, in which case this is the 3rd to
 * the last field.
 */

#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <limits.h>
#include <linux/limits.h>

int latestcpu(pid_t pid)
{
	char buf[PATH_MAX];
	int fd;
	int cpu = -1;

	snprintf(buf, sizeof(buf), "/proc/%d/stat", pid);
	fd = open(buf, O_RDONLY);
	buf[0] = 0;     /* in case fd < 0 and read() is no-op */
	read(fd, buf, sizeof(buf));
	close(fd);
	sscanf(buf, "%*u %*s %*s %*u %*u %*u %*u %*u %*u %*u "
		    "%*u %*u %*u %*u %*u %*u %*u %*u %*u %*u "
		    "%*u %*u %*u %*u %*u %*u %*u %*u %*u %*u "
		    "%*u %*u %*u %*u %*u %*u %*u %*u %u", /* 39th field */
		    &cpu);
	return cpu;
}

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401