Drive your load average arbitrarily high

spin.c

/*-
 * Copyright (c) 2021 Dag-Erling Smørgrav
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * This program will drive your load average arbitrarily high while
 * consuming a minimum of memory.  In theory, the load average sould reach
 * 90%-95% of the number of threads specified on the command line (-n).
 * The percentage will be lower if you increase the sleep time (-s).
 * Memory usage should be asymptotical to one page (4096 kB) per thread.
 * Note that it may take a while (possibly minutes) for the load to reach
 * the target.
 */

#include <err.h>
#include <limits.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>

static unsigned long nthreads = 10;
static unsigned long sleeptime = 1;

static struct timespec rqtp = { .tv_sec = 0, .tv_nsec = 1 };

static void *spin(void *) __attribute__((__noreturn__));
static void *spin(void *arg __attribute__((__unused__)))
{
	for (;;) {
		clock_nanosleep(CLOCK_MONOTONIC, 0, &rqtp, NULL);
	}
}

static unsigned long optul(int opt, const char *optarg,
    unsigned long min, unsigned long max)
{
	unsigned long num;

	if (*optarg == '+') {
		optarg++;
	}
	while (*optarg != '\0') {
		if (*optarg < '0' || *optarg > '9') {
			errx(1, "-%c: invalid number", opt);
		}
		if (num * 10 + *optarg - '0' < num) {
			errx(1, "-%c: overflow", opt);
		}
		num = num * 10 + *optarg - '0';
		optarg++;
	}
	if (num < min || num > max) {
		errx(1, "-%c: value must be between %lu and %lu", opt, min, max);
	}
	return num;
}

static void usage(const char *arg0)
{
	for (const char *p = arg0; *p != '\0'; p++) {
		if (*p == '/') {
			arg0 = p + 1;
		}
	}
	fprintf(stderr, "usage: %s [-n nthreads] [-t sleeptime]\n", arg0);
	exit(1);
}

int main(int argc, char *argv[])
{
	pthread_attr_t attr;
	pthread_t thr;
	int opt;

	while ((opt = getopt(argc, argv, "n:s:")) != -1) {
		switch (opt) {
		case 'n':
			nthreads = optul(opt, optarg, 1, ULONG_MAX);
			break;
		case 's':
			sleeptime = optul(opt, optarg, 1, ULONG_MAX);
			break;
		default:
			usage(argv[0]);
		}
	}
	if (argc > optind) {
		usage(argv[0]);
	}
	argc -= optind;
	argv += optind;
	pthread_attr_init(&attr);
	pthread_attr_setstacksize(&attr, 64); // will be rounded up
	for (unsigned long i = 0; i < nthreads; i++) {
		pthread_create(&thr, &attr, spin, NULL);
	}
	pthread_join(thr, NULL);
	exit(0);
}