Re: BFS vs. mainline scheduler benchmarks and measurements

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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On Tue, 2009-09-08 at 11:13 +0200, Jens Axboe wrote:
> And here's a newer version.

I tinkered a bit with your proglet and finally found the problem.

You used a single pipe per child, this means the loop in run_child()
would consume what it just wrote out until it got force preempted by the
parent which would also get woken.

This results in the child spinning a while (its full quota) and only
reporting the last timestamp to the parent.

Since consumer (parent) is a single thread the program basically
measures the worst delay in a thundering herd wakeup of N children.

The below version yields:

idle

[root@opteron sched]# ./latt -c8 sleep 30
Entries: 664 (clients=8)

Averages:
------------------------------
        Max           128 usec
        Avg            26 usec
        Stdev          16 usec


make -j4

[root@opteron sched]# ./latt -c8 sleep 30
Entries: 648 (clients=8)                 

Averages:
------------------------------
        Max         20861 usec
        Avg          3763 usec
        Stdev        4637 usec


Mike's patch, make -j4

[root@opteron sched]# ./latt -c8 sleep 30
Entries: 648 (clients=8)

Averages:
------------------------------
        Max         17854 usec
        Avg          6298 usec
        Stdev        4735 usec


[-- Attachment #2: latt.c --]
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/*
 * Simple latency tester that combines multiple processes.
 *
 * Compile: gcc -Wall -O2 -D_GNU_SOURCE -lrt -lm -o latt latt.c
 *
 * Run with: latt -c8 'program --args'
 *
 * Options:
 *
 *	-cX	Use X number of clients
 *	-fX	Use X msec as the minimum sleep time for the parent
 *	-tX	Use X msec as the maximum sleep time for the parent
 *	-v	Print all delays as they are logged
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <getopt.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <time.h>
#include <math.h>
#include <poll.h>
#include <pthread.h>


/*
 * In msecs
 */
static unsigned int min_delay = 100;
static unsigned int max_delay = 500;
static unsigned int clients = 1;
static unsigned int verbose;

#define MAX_CLIENTS		512

struct stats
{
	double n, mean, M2, max;
};

static void update_stats(struct stats *stats, unsigned long long val)
{
	double delta, x = val;

	stats->n++;
	delta = x - stats->mean;
	stats->mean += delta / stats->n;
	stats->M2 += delta*(x - stats->mean);

	if (stats->max < x)
		stats->max = x;
}

static unsigned long nr_stats(struct stats *stats)
{
	return stats->n;
}

static double max_stats(struct stats *stats)
{
	return stats->max;
}

static double avg_stats(struct stats *stats)
{
	return stats->mean;
}

/*
 * http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance
 *
 *       (\Sum n_i^2) - ((\Sum n_i)^2)/n
 * s^2 = -------------------------------
 *                  n - 1
 *
 * http://en.wikipedia.org/wiki/Stddev
 */
static double stddev_stats(struct stats *stats)
{
	double variance = stats->M2 / (stats->n - 1);

	return sqrt(variance);
}

/*
 * The std dev of the mean is related to the std dev by:
 *
 *             s
 * s_mean = -------
 *          sqrt(n)
 *
 */
static double stddev_mean_stats(struct stats *stats)
{
	double variance = stats->M2 / (stats->n - 1);
	double variance_mean = variance / stats->n;

	return sqrt(variance_mean);
}

struct stats delay_stats;

static int pipes[MAX_CLIENTS*2][2];

static pid_t app_pid;

#define CLOCKSOURCE		CLOCK_MONOTONIC

struct sem {
	pthread_mutex_t lock;
	pthread_cond_t cond;
	int value;
	int waiters;
};

static void init_sem(struct sem *sem)
{
	pthread_mutexattr_t attr;
	pthread_condattr_t cond;

	pthread_mutexattr_init(&attr);
	pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
	pthread_condattr_init(&cond);
	pthread_condattr_setpshared(&cond, PTHREAD_PROCESS_SHARED);
	pthread_cond_init(&sem->cond, &cond);
	pthread_mutex_init(&sem->lock, &attr);

	sem->value = 0;
	sem->waiters = 0;
}

static void sem_down(struct sem *sem)
{
	pthread_mutex_lock(&sem->lock);

	while (!sem->value) {
		sem->waiters++;
		pthread_cond_wait(&sem->cond, &sem->lock);
		sem->waiters--;
	}

	sem->value--;
	pthread_mutex_unlock(&sem->lock);
}

static void sem_up(struct sem *sem)
{
	pthread_mutex_lock(&sem->lock);
	if (!sem->value && sem->waiters)
		pthread_cond_signal(&sem->cond);
	sem->value++;
	pthread_mutex_unlock(&sem->lock);
}

static int parse_options(int argc, char *argv[])
{
	struct option l_opts[] = {
		{ "min-delay", 	1, 	NULL,	'f' },
		{ "max-delay",	1,	NULL,	't' },
		{ "clients",	1,	NULL,	'c' },
		{ "verbose",	1,	NULL,	'v' }
	};
	int c, res, index = 0;

	while ((c = getopt_long(argc, argv, "f:t:c:v", l_opts, &res)) != -1) {
		index++;
		switch (c) {
			case 'f':
				min_delay = atoi(optarg);
				break;
			case 't':
				max_delay = atoi(optarg);
				break;
			case 'c':
				clients = atoi(optarg);
				if (clients > MAX_CLIENTS)
					clients = MAX_CLIENTS;
				break;
			case 'v':
				verbose = 1;
				break;
		}
	}

	return index + 1;
}

static pid_t fork_off(const char *app)
{
	pid_t pid;

	pid = fork();
	if (pid)
		return pid;

	exit(system(app));
}

static unsigned long usec_since(struct timespec *start, struct timespec *end)
{
	unsigned long long s, e;

	s = start->tv_sec * 1000000000ULL + start->tv_nsec;
	e =   end->tv_sec * 1000000000ULL +   end->tv_nsec;

	return (e - s) / 1000;
}

static void log_delay(unsigned long delay)
{
	if (verbose) {
		fprintf(stderr, "log delay %8lu usec\n", delay);
		fflush(stderr);
	}

	update_stats(&delay_stats, delay);
}

/*
 * Reads a timestamp (which is ignored, it's just a wakeup call), and replies
 * with the timestamp of when we saw it
 */
static void run_child(int *in, int *out, struct sem *sem)
{
	struct timespec ts;

	if (verbose) {
		fprintf(stderr, "present: %d\n", getpid());
		fflush(stderr);
	}

	sem_up(sem);

	do {
		int ret;

		ret = read(in[0], &ts, sizeof(ts));
		if (ret <= 0)
			break;

		if (ret != sizeof(ts))
			printf("bugger3\n");

		clock_gettime(CLOCKSOURCE, &ts);

		ret = write(out[1], &ts, sizeof(ts));
		if (ret <= 0)
			break;

		if (ret != sizeof(ts))
			printf("bugger4\n");

		if (verbose) {
			fprintf(stderr, "alife: %d\n", getpid());
			fflush(stderr);
		}
	} while (1);
}

/*
 * Do a random sleep between min and max delay
 */
static void do_rand_sleep(void)
{
	unsigned int msecs;

	msecs = min_delay + ((float) max_delay * (rand() / (RAND_MAX + 1.0)));
	if (verbose) {
		fprintf(stderr, "sleeping for: %u msec\n", msecs);
		fflush(stderr);
	}
	usleep(msecs * 1000);
}

static void kill_connection(void)
{
	int i;

	for (i = 0; i < 2*clients; i++) {
		if (pipes[i][0] != -1) {
			close(pipes[i][0]);
			pipes[i][0] = -1;
		}
		if (pipes[i][1] != -1) {
			close(pipes[i][1]);
			pipes[i][1] = -1;
		}
	}
}

static int __write_ts(int i, struct timespec *ts)
{
	int fd = pipes[2*i][1];

	clock_gettime(CLOCKSOURCE, ts);

	return write(fd, ts, sizeof(*ts)) != sizeof(*ts);
}

static long __read_ts(int i, struct timespec *ts)
{
	int fd = pipes[2*i+1][0];
	struct timespec t;

	if (read(fd, &t, sizeof(t)) != sizeof(t))
		return -1;

	log_delay(usec_since(ts, &t));

	return 0;
}

static int read_ts(struct pollfd *pfd, unsigned int nr, struct timespec *ts)
{
	unsigned int i;

	for (i = 0; i < clients; i++) {
		if (pfd[i].revents & (POLLERR | POLLHUP | POLLNVAL))
			return -1L;
		if (pfd[i].revents & POLLIN) {
			pfd[i].events = 0;
			if (__read_ts(i, &ts[i]))
				return -1L;
			nr--;
		}
		if (!nr)
			break;
	}

	return 0;
}

static int app_has_exited(void)
{
	int ret, status;

	/*
	 * If our app has exited, stop
	 */
	ret = waitpid(app_pid, &status, WNOHANG);
	if (ret < 0) {
		perror("waitpid");
		return 1;
	} else if (ret == app_pid &&
		   (WIFSIGNALED(status) || WIFEXITED(status))) {
		return 1;
	}

	return 0;
}

/*
 * While our given app is running, send a timestamp to each client and
 * log the maximum latency for each of them to wakeup and reply
 */
static void run_parent(pid_t *cpids)
{
	struct pollfd *ipfd;
	int do_exit = 0, i;
	struct timespec *t1;

	t1 = malloc(sizeof(struct timespec) * clients);
	ipfd = malloc(sizeof(struct pollfd) * clients);

	srand(1234);

	do {
		unsigned long delay;
		unsigned pending_events;

		do_rand_sleep();

		if (app_has_exited())
			break;

		for (i = 0; i < clients; i++) {
			ipfd[i].fd = pipes[2*i+1][0];
			ipfd[i].events = POLLIN;
		}

		/*
		 * Write wakeup calls
		 */
		for (i = 0; i < clients; i++) {
			if (verbose) {
				fprintf(stderr, "waking: %d\n", cpids[i]);
				fflush(stderr);
			}

			if (__write_ts(i, t1+i)) {
				do_exit = 1;
				break;
			}
		}

		if (do_exit)
			break;

		/*
		 * Poll and read replies
		 */
		pending_events = clients;
		while (pending_events) {
			int evts = poll(ipfd, clients, 0);

			if (evts < 0) {
				do_exit = 1;
				break;
			} else if (!evts) {
				/* printf("bugger2\n"); */
				continue;
			}

			if (read_ts(ipfd, evts, t1)) {
				do_exit = 1;
				break;
			}

			pending_events -= evts;
		}
	} while (!do_exit);

	free(t1);
	free(ipfd);
	kill_connection();
}

static void run_test(void)
{
	struct sem *sem;
	pid_t *cpids;
	int i, status;

	sem = mmap(NULL, sizeof(*sem), PROT_READ|PROT_WRITE,
			MAP_SHARED | MAP_ANONYMOUS, 0, 0);
	if (sem == MAP_FAILED) {
		perror("mmap");
		return;
	}

	init_sem(sem);

	for (i = 0; i < 2*clients; i++) {
		if (pipe(pipes[i])) {
			perror("pipe");
			return;
		}
	}

	cpids = malloc(sizeof(pid_t) * clients);

	for (i = 0; i < clients; i++) {
		cpids[i] = fork();
		if (cpids[i]) {
			sem_down(sem);
			continue;
		}

		run_child(pipes[2*i], pipes[2*i+1], sem);
		exit(0);
	}

	run_parent(cpids);

	for (i = 0; i < clients; i++)
		kill(cpids[i], SIGQUIT);
	for (i = 0; i < clients; i++)
		waitpid(cpids[i], &status, 0);

	free(cpids);
	munmap(sem, sizeof(*sem));
}

static void handle_sigint(int sig)
{
	kill(app_pid, SIGINT);
}

int main(int argc, char *argv[])
{
	int app_offset, off;
	char app[256];

	off = 0;
	app_offset = parse_options(argc, argv);
	while (app_offset < argc) {
		if (off) {
			app[off] = ' ';
			off++;
		}
		off += sprintf(app + off, "%s", argv[app_offset]);
		app_offset++;
	}

	signal(SIGINT, handle_sigint);

	/*
	 * Start app and start logging latencies
	 */
	app_pid = fork_off(app);
	run_test();

	printf("Entries: %lu (clients=%d)\n", nr_stats(&delay_stats), clients);
	printf("\nAverages:\n");
	printf("------------------------------\n");
	printf("\tMax\t %8.0f usec\n", max_stats(&delay_stats));
	printf("\tAvg\t %8.0f usec\n", avg_stats(&delay_stats));
	printf("\tStdev\t %8.0f usec\n", stddev_stats(&delay_stats));

	return 0;
}