zero-to-n.c   [plain text]

/*
 * Copyright (c) 2009 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 * 
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
#include <unistd.h>
#include <stdio.h>
#include <math.h>
#include <sys/kdebug.h>
#include <stdlib.h>
#include <pthread.h>
#include <errno.h>
#include <err.h>
#include <string.h>
#include <assert.h>
#include <sysexits.h>
#include <sys/sysctl.h>
#include <getopt.h>

#include <spawn.h>
#include <spawn_private.h>
#include <sys/spawn_internal.h>
#include <mach-o/dyld.h>

#include <libkern/OSAtomic.h>

#include <mach/mach_time.h>
#include <mach/mach.h>
#include <mach/task.h>
#include <mach/semaphore.h>

#include <pthread/qos_private.h>

typedef enum wake_type { WAKE_BROADCAST_ONESEM, WAKE_BROADCAST_PERTHREAD, WAKE_CHAIN, WAKE_HOP } wake_type_t;
typedef enum my_policy_type { MY_POLICY_REALTIME, MY_POLICY_TIMESHARE, MY_POLICY_FIXEDPRI } my_policy_type_t;

#define mach_assert_zero(error)        do { if ((error) != 0) { fprintf(stderr, "[FAIL] error %d (%s) ", (error), mach_error_string(error)); assert(error == 0); } } while (0)
#define mach_assert_zero_t(tid, error) do { if ((error) != 0) { fprintf(stderr, "[FAIL] Thread %d error %d (%s) ", (tid), (error), mach_error_string(error)); assert(error == 0); } } while (0)
#define assert_zero_t(tid, error)      do { if ((error) != 0) { fprintf(stderr, "[FAIL] Thread %d error %d ", (tid), (error)); assert(error == 0); } } while (0)

#define CONSTRAINT_NANOS	(20000000ll)	/* 20 ms */
#define COMPUTATION_NANOS	(10000000ll)	/* 10 ms */
#define TRACEWORTHY_NANOS	(10000000ll)	/* 10 ms */

#if DEBUG
#define debug_log(args...) printf(args)
#else
#define debug_log(args...) do { } while(0)
#endif

/* Declarations */
static void*                    worker_thread(void *arg);
static void                     usage();
static int                      thread_setup(uint32_t my_id);
static my_policy_type_t         parse_thread_policy(const char *str);
static void                     selfexec_with_apptype(int argc, char *argv[]);
static void                     parse_args(int argc, char *argv[]);

/* Global variables (general) */
static uint32_t                 g_numcpus;
static uint32_t                 g_numthreads;
static wake_type_t              g_waketype;
static policy_t                 g_policy;
static uint32_t                 g_iterations;
static struct mach_timebase_info g_mti;
static semaphore_t              g_main_sem;
static uint64_t                *g_thread_endtimes_abs;
static volatile uint32_t        g_done_threads;
static boolean_t                g_verbose       = FALSE;
static boolean_t                g_do_affinity   = FALSE;
static uint64_t                 g_starttime_abs;
static uint32_t                 g_iteration_sleeptime_us = 0;

/* Threshold for dropping a 'bad run' tracepoint */
static uint64_t                 g_traceworthy_latency_ns = TRACEWORTHY_NANOS;

/* Have we re-execed to set apptype? */
static boolean_t                g_seen_apptype = FALSE;

/* usleep in betweeen iterations */
static boolean_t                g_do_sleep      = TRUE;

/* Every thread spins until all threads have checked in */
static boolean_t                g_do_all_spin = FALSE;

/* One randomly chosen thread holds up the train for a certain duration. */
static boolean_t                g_do_one_long_spin = FALSE;
static uint32_t                 g_one_long_spin_id = 0;
static uint64_t                 g_one_long_spin_length_abs = 0;
static uint64_t                 g_one_long_spin_length_ns = 0;

/* Each thread spins for a certain duration after waking up before blocking again. */
static boolean_t                g_do_each_spin = FALSE;
static uint64_t                 g_each_spin_duration_abs = 0;
static uint64_t                 g_each_spin_duration_ns = 0;

/* Global variables (broadcast) */
static semaphore_t              g_broadcastsem;
static semaphore_t              g_leadersem;
static semaphore_t              g_readysem;
static semaphore_t              g_donesem;

/* Global variables (chain) */
static semaphore_t             *g_semarr;

static uint64_t
abs_to_nanos(uint64_t abstime)
{
	return (uint64_t)(abstime * (((double)g_mti.numer) / ((double)g_mti.denom)));
}

static uint64_t
nanos_to_abs(uint64_t ns)
{
	return (uint64_t)(ns * (((double)g_mti.denom) / ((double)g_mti.numer)));
}

/*
 * Figure out what thread policy to use 
 */
static my_policy_type_t
parse_thread_policy(const char *str)
{
	if (strcmp(str, "timeshare") == 0) {
		return MY_POLICY_TIMESHARE;
	} else if (strcmp(str, "realtime") == 0) {
		return MY_POLICY_REALTIME;
	} else if (strcmp(str, "fixed") == 0) {
		return MY_POLICY_FIXEDPRI;
	} else {
		errx(EX_USAGE, "Invalid thread policy \"%s\"", str);
	}
}

/*
 * Figure out what wakeup pattern to use
 */
static wake_type_t
parse_wakeup_pattern(const char *str) 
{
	if (strcmp(str, "chain") == 0) {
		return WAKE_CHAIN;
	} else if (strcmp(str, "hop") == 0) {
		return WAKE_HOP;
	} else if (strcmp(str, "broadcast-single-sem") == 0) {
		return WAKE_BROADCAST_ONESEM;
	} else if (strcmp(str, "broadcast-per-thread") == 0) {
		return WAKE_BROADCAST_PERTHREAD;
	} else {
		errx(EX_USAGE, "Invalid wakeup pattern \"%s\"", str);
	}
}

/*
 * Set policy
 */
static int
thread_setup(uint32_t my_id)
{
	kern_return_t kr;
	errno_t ret;
	thread_time_constraint_policy_data_t pol;

	switch (g_policy) {
		case MY_POLICY_TIMESHARE:
			break;
		case MY_POLICY_REALTIME:
			/* Hard-coded realtime parameters (similar to what Digi uses) */
			pol.period      = 100000;
			pol.constraint  = (uint32_t) nanos_to_abs(CONSTRAINT_NANOS);
			pol.computation = (uint32_t) nanos_to_abs(COMPUTATION_NANOS);
			pol.preemptible = 0; /* Ignored by OS */

			kr = thread_policy_set(mach_thread_self(), THREAD_TIME_CONSTRAINT_POLICY,
			                       (thread_policy_t) &pol, THREAD_TIME_CONSTRAINT_POLICY_COUNT);
			mach_assert_zero_t(my_id, kr);
			break;
		case MY_POLICY_FIXEDPRI:
			ret = pthread_set_fixedpriority_self();
			if (ret) errc(EX_OSERR, ret, "pthread_set_fixedpriority_self");
			break;
		default:
			errx(EX_USAGE, "invalid policy type %d", g_policy);
	}

	if (g_do_affinity) {
		thread_affinity_policy_data_t affinity;

		affinity.affinity_tag = my_id % 2;

		kr = thread_policy_set(mach_thread_self(), THREAD_AFFINITY_POLICY,
		                       (thread_policy_t)&affinity, THREAD_AFFINITY_POLICY_COUNT);
		mach_assert_zero_t(my_id, kr);
	}

	return 0;
}

/*
 * Wait for a wakeup, potentially wake up another of the "0-N" threads,
 * and notify the main thread when done.
 */
static void*
worker_thread(void *arg)
{
	uint32_t my_id = (uint32_t)(uintptr_t)arg;
	kern_return_t kr;

	volatile double x = 0.0;
	volatile double y = 0.0;

	/* Set policy and so forth */
	thread_setup(my_id);

	for (uint32_t i = 0; i < g_iterations; i++) {
		if (my_id == 0) {
			/*
			 * Leader thread either wakes everyone up or starts the chain going.
			 */

			/* Give the worker threads undisturbed time to finish before waiting on them */
			if (g_do_sleep)
				usleep(g_iteration_sleeptime_us);

			debug_log("%d Leader thread wait for ready\n", i);

			/*
			 * Wait for everyone else to declare ready
			 * Is there a better way to do this that won't interfere with the rest of the chain?
			 * TODO: Invent 'semaphore wait for N signals'
			 */

			for (uint32_t j = 0 ; j < g_numthreads - 1; j++) {
				kr = semaphore_wait(g_readysem);
				mach_assert_zero_t(my_id, kr);
			}

			debug_log("%d Leader thread wait\n", i);

			/* Signal main thread and wait for start of iteration */

			kr = semaphore_wait_signal(g_leadersem, g_main_sem);
			mach_assert_zero_t(my_id, kr);

			g_thread_endtimes_abs[my_id] = mach_absolute_time();

			debug_log("%d Leader thread go\n", i);

			assert_zero_t(my_id, g_done_threads);

			switch (g_waketype) {
			case WAKE_BROADCAST_ONESEM:
				kr = semaphore_signal_all(g_broadcastsem);
				mach_assert_zero_t(my_id, kr);
				break;
			case WAKE_BROADCAST_PERTHREAD:
				for (uint32_t j = 1; j < g_numthreads; j++) {
					kr = semaphore_signal(g_semarr[j]);
					mach_assert_zero_t(my_id, kr);
				}
				break;
			case WAKE_CHAIN:
				kr = semaphore_signal(g_semarr[my_id + 1]);
				mach_assert_zero_t(my_id, kr);
				break;
			case WAKE_HOP:
				kr = semaphore_wait_signal(g_donesem, g_semarr[my_id + 1]);
				mach_assert_zero_t(my_id, kr);
				break;
			}
		} else {
			/*
			 * Everyone else waits to be woken up,
			 * records when she wakes up, and possibly
			 * wakes up a friend.
			 */
			switch(g_waketype)  {
			case WAKE_BROADCAST_ONESEM:
				kr = semaphore_wait_signal(g_broadcastsem, g_readysem);
				mach_assert_zero_t(my_id, kr);

				g_thread_endtimes_abs[my_id] = mach_absolute_time();
				break;

			case WAKE_BROADCAST_PERTHREAD:
				kr = semaphore_wait_signal(g_semarr[my_id], g_readysem);
				mach_assert_zero_t(my_id, kr);

				g_thread_endtimes_abs[my_id] = mach_absolute_time();
				break;

			case WAKE_CHAIN:
				kr = semaphore_wait_signal(g_semarr[my_id], g_readysem);
				mach_assert_zero_t(my_id, kr);

				/* Signal the next thread *after* recording wake time */

				g_thread_endtimes_abs[my_id] = mach_absolute_time();

				if (my_id < (g_numthreads - 1)) {
					kr = semaphore_signal(g_semarr[my_id + 1]);
					mach_assert_zero_t(my_id, kr);
				}

				break;

			case WAKE_HOP:
				kr = semaphore_wait_signal(g_semarr[my_id], g_readysem);
				mach_assert_zero_t(my_id, kr);

				/* Signal the next thread *after* recording wake time */

				g_thread_endtimes_abs[my_id] = mach_absolute_time();

				if (my_id < (g_numthreads - 1)) {
					kr = semaphore_wait_signal(g_donesem, g_semarr[my_id + 1]);
					mach_assert_zero_t(my_id, kr);
				} else {
					kr = semaphore_signal_all(g_donesem);
					mach_assert_zero_t(my_id, kr);
				}

				break;
			}
		}

		debug_log("Thread %p woke up for iteration %d.\n", pthread_self(), i);

		if (g_do_one_long_spin && g_one_long_spin_id == my_id) {
			/* One randomly chosen thread holds up the train for a while. */

			uint64_t endspin = g_starttime_abs + g_one_long_spin_length_abs;
			while (mach_absolute_time() < endspin) {
				y = y + 1.5 + x;
				x = sqrt(y);
			}
		}

		if (g_do_each_spin) {
			/* Each thread spins for a certain duration after waking up before blocking again. */

			uint64_t endspin = mach_absolute_time() + g_each_spin_duration_abs;
			while (mach_absolute_time() < endspin) {
				y = y + 1.5 + x;
				x = sqrt(y);
			}
		}

		int32_t new = OSAtomicIncrement32((volatile int32_t *)&g_done_threads);
		(void)new;

		debug_log("Thread %p new value is %d, iteration %d\n", pthread_self(), new, i);

		if (g_do_all_spin) {
			/* Everyone spins until the last thread checks in. */

			while (g_done_threads < g_numthreads) {
				y = y + 1.5 + x;
				x = sqrt(y);
			}
		}

		debug_log("Thread %p done spinning, iteration %d\n", pthread_self(), i);
	}

	if (my_id == 0) {
		/* Give the worker threads undisturbed time to finish before waiting on them */
		if (g_do_sleep)
			usleep(g_iteration_sleeptime_us);

		/* Wait for the worker threads to finish */
		for (uint32_t i = 0 ; i < g_numthreads - 1; i++) {
			kr = semaphore_wait(g_readysem);
			mach_assert_zero_t(my_id, kr);
		}

		/* Tell everyone and the main thread that the last iteration is done */
		debug_log("%d Leader thread done\n", i);

		kr = semaphore_signal_all(g_main_sem);
		mach_assert_zero_t(my_id, kr);
	} else {
		/* Hold up thread teardown so it doesn't affect the last iteration */
		kr = semaphore_wait_signal(g_main_sem, g_readysem);
		mach_assert_zero_t(my_id, kr);
	}

	return 0;
}

/*
 * Given an array of uint64_t values, compute average, max, min, and standard deviation
 */
static void
compute_stats(uint64_t *values, uint64_t count, float *averagep, uint64_t *maxp, uint64_t *minp, float *stddevp)
{
	uint32_t i;
	uint64_t _sum = 0;
	uint64_t _max = 0;
	uint64_t _min = UINT64_MAX;
	float	 _avg = 0;
	float 	 _dev = 0;

	for (i = 0; i < count; i++) {
		_sum += values[i];
		_max = values[i] > _max ? values[i] : _max;
		_min = values[i] < _min ? values[i] : _min;
	}

	_avg = ((float)_sum) / ((float)count);
	
	_dev = 0;
	for (i = 0; i < count; i++) {
		_dev += powf((((float)values[i]) - _avg), 2);
	}
	
	_dev /= count;
	_dev = sqrtf(_dev);

	*averagep = _avg;
	*maxp = _max;
	*minp = _min;
	*stddevp = _dev;
}

int
main(int argc, char **argv)
{
	errno_t ret;
	kern_return_t kr;

	pthread_t	*threads;
	uint64_t	*worst_latencies_ns;
	uint64_t	*worst_latencies_from_first_ns;
	uint64_t	max, min;
	float		avg, stddev;

	for (int i = 0; i < argc; i++)
		if (strcmp(argv[i], "--switched_apptype") == 0)
			g_seen_apptype = TRUE;

	if (!g_seen_apptype)
		selfexec_with_apptype(argc, argv);

	parse_args(argc, argv);

	srand((unsigned int)time(NULL));

	mach_timebase_info(&g_mti);

	size_t ncpu_size = sizeof(g_numcpus);
	ret = sysctlbyname("hw.ncpu", &g_numcpus, &ncpu_size, NULL, 0);
	if (ret) err(EX_OSERR, "Failed sysctlbyname(hw.ncpu)");

	if (g_do_each_spin)
		g_each_spin_duration_abs = nanos_to_abs(g_each_spin_duration_ns);

	/* Configure the long-spin thread to take up half of its computation */
	if (g_do_one_long_spin) {
		g_one_long_spin_length_ns = COMPUTATION_NANOS / 2;
		g_one_long_spin_length_abs = nanos_to_abs(g_one_long_spin_length_ns);
	}

	/* Estimate the amount of time the cleanup phase needs to back off */
	g_iteration_sleeptime_us = g_numthreads * 20;

	uint32_t threads_per_core = (g_numthreads / g_numcpus) + 1;
	if (g_do_each_spin)
		g_iteration_sleeptime_us += threads_per_core * (g_each_spin_duration_ns / NSEC_PER_USEC);
	if (g_do_one_long_spin)
		g_iteration_sleeptime_us += g_one_long_spin_length_ns / NSEC_PER_USEC;

	/* Arrays for threads and their wakeup times */
	threads = (pthread_t*) valloc(sizeof(pthread_t) * g_numthreads);
	assert(threads);

	size_t endtimes_size = sizeof(uint64_t) * g_numthreads;

	g_thread_endtimes_abs = (uint64_t*) valloc(endtimes_size);
	assert(g_thread_endtimes_abs);

	/* Ensure the allocation is pre-faulted */
	ret = memset_s(g_thread_endtimes_abs, endtimes_size, 0, endtimes_size);
	if (ret) errc(EX_OSERR, ret, "memset_s endtimes");

	size_t latencies_size = sizeof(uint64_t) * g_iterations;

	worst_latencies_ns = (uint64_t*) valloc(latencies_size);
	assert(worst_latencies_ns);

	/* Ensure the allocation is pre-faulted */
	ret = memset_s(worst_latencies_ns, latencies_size, 0, latencies_size);
	if (ret) errc(EX_OSERR, ret, "memset_s latencies");

	worst_latencies_from_first_ns = (uint64_t*) valloc(latencies_size);
	assert(worst_latencies_from_first_ns);

	/* Ensure the allocation is pre-faulted */
	ret = memset_s(worst_latencies_from_first_ns, latencies_size, 0, latencies_size);
	if (ret) errc(EX_OSERR, ret, "memset_s latencies_from_first");

	kr = semaphore_create(mach_task_self(), &g_main_sem, SYNC_POLICY_FIFO, 0);
	mach_assert_zero(kr);

	/* Either one big semaphore or one per thread */
	if (g_waketype == WAKE_CHAIN ||
	    g_waketype == WAKE_BROADCAST_PERTHREAD ||
	    g_waketype == WAKE_HOP) {

		g_semarr = valloc(sizeof(semaphore_t) * g_numthreads);
		assert(g_semarr);

		for (uint32_t i = 0; i < g_numthreads; i++) {
			kr = semaphore_create(mach_task_self(), &g_semarr[i], SYNC_POLICY_FIFO, 0);
			mach_assert_zero(kr);
		}

		g_leadersem = g_semarr[0];
	} else {
		kr = semaphore_create(mach_task_self(), &g_broadcastsem, SYNC_POLICY_FIFO, 0);
		mach_assert_zero(kr);
		kr = semaphore_create(mach_task_self(), &g_leadersem, SYNC_POLICY_FIFO, 0);
		mach_assert_zero(kr);
	}

	if (g_waketype == WAKE_HOP) {
		kr = semaphore_create(mach_task_self(), &g_donesem, SYNC_POLICY_FIFO, 0);
		mach_assert_zero(kr);
	}

	kr = semaphore_create(mach_task_self(), &g_readysem, SYNC_POLICY_FIFO, 0);
	mach_assert_zero(kr);

	/* Create the threads */
	g_done_threads = 0;
	for (uint32_t i = 0; i < g_numthreads; i++) {
		ret = pthread_create(&threads[i], NULL, worker_thread, (void*)(uintptr_t)i);
		if (ret) errc(EX_OSERR, ret, "pthread_create %d", i);
	}

	ret = setpriority(PRIO_DARWIN_ROLE, 0, PRIO_DARWIN_ROLE_UI_FOCAL);
	if (ret) errc(EX_OSERR, ret, "setpriority");

	thread_setup(0);

	/* Let everyone get settled */
	kr = semaphore_wait(g_main_sem);
	mach_assert_zero(kr);

	/* Give the system a bit more time to settle */
	if (g_do_sleep)
		usleep(g_iteration_sleeptime_us);

	/* Go! */
	for (uint32_t i = 0; i < g_iterations; i++) {
		uint32_t j;
		uint64_t worst_abs = 0, best_abs = UINT64_MAX;

		if (g_do_one_long_spin)
			g_one_long_spin_id = (uint32_t)rand() % g_numthreads;

		debug_log("%d Main thread reset\n", i);

		g_done_threads = 0;
		OSMemoryBarrier();

		g_starttime_abs = mach_absolute_time();

		/* Fire them off and wait for worker threads to finish */
		kr = semaphore_wait_signal(g_main_sem, g_leadersem);
		mach_assert_zero(kr);

		debug_log("%d Main thread return\n", i);

		/*
		 * We report the worst latencies relative to start time
		 * and relative to the lead worker thread.
		 */
		for (j = 0; j < g_numthreads; j++) {
			uint64_t latency_abs;

			latency_abs = g_thread_endtimes_abs[j] - g_starttime_abs;
			worst_abs = worst_abs < latency_abs ? latency_abs : worst_abs;
		}
	
		worst_latencies_ns[i] = abs_to_nanos(worst_abs);

		worst_abs = 0;
		for (j = 1; j < g_numthreads; j++) {
			uint64_t latency_abs;
		
			latency_abs = g_thread_endtimes_abs[j] - g_thread_endtimes_abs[0];
			worst_abs = worst_abs < latency_abs ? latency_abs : worst_abs;
			best_abs = best_abs > latency_abs ? latency_abs : best_abs;
		}

		worst_latencies_from_first_ns[i] = abs_to_nanos(worst_abs);

		/*
		 * In the event of a bad run, cut a trace point.
		 */
		if (worst_latencies_from_first_ns[i] > g_traceworthy_latency_ns) {
			/* Ariadne's ad-hoc test signpost */
			kdebug_trace(ARIADNEDBG_CODE(0, 0), worst_latencies_from_first_ns[i], g_traceworthy_latency_ns, 0, 0);

			if (g_verbose)
				printf("Worst on this round was %.2f us.\n", ((float)worst_latencies_from_first_ns[i]) / 1000.0);
		}

		/* Give the system a bit more time to settle */
		if (g_do_sleep)
			usleep(g_iteration_sleeptime_us);
	}

	/* Rejoin threads */
	for (uint32_t i = 0; i < g_numthreads; i++) {
		ret = pthread_join(threads[i], NULL);
		if (ret) errc(EX_OSERR, ret, "pthread_join %d", i);
	}

	compute_stats(worst_latencies_ns, g_iterations, &avg, &max, &min, &stddev);
	printf("Results (from a stop):\n");
	printf("Max:\t\t%.2f us\n", ((float)max) / 1000.0);
	printf("Min:\t\t%.2f us\n", ((float)min) / 1000.0);
	printf("Avg:\t\t%.2f us\n", avg / 1000.0);
	printf("Stddev:\t\t%.2f us\n", stddev / 1000.0);

	putchar('\n');

	compute_stats(worst_latencies_from_first_ns, g_iterations, &avg, &max, &min, &stddev);
	printf("Results (relative to first thread):\n");
	printf("Max:\t\t%.2f us\n", ((float)max) / 1000.0);
	printf("Min:\t\t%.2f us\n", ((float)min) / 1000.0);
	printf("Avg:\t\t%.2f us\n", avg / 1000.0);
	printf("Stddev:\t\t%.2f us\n", stddev / 1000.0);

#if 0
	for (uint32_t i = 0; i < g_iterations; i++) {
		printf("Iteration %d: %f us\n", i, worst_latencies_ns[i] / 1000.0);
	}
#endif

	free(threads);
	free(g_thread_endtimes_abs);
	free(worst_latencies_ns);
	free(worst_latencies_from_first_ns);

	return 0;
}

/*
 * WARNING: This is SPI specifically intended for use by launchd to start UI
 * apps. We use it here for a test tool only to opt into QoS using the same
 * policies. Do not use this outside xnu or libxpc/launchd.
 */
static void
selfexec_with_apptype(int argc, char *argv[])
{
	int ret;
	posix_spawnattr_t attr;
	extern char **environ;
	char *new_argv[argc + 1 + 1 /* NULL */];
	int i;
	char prog[PATH_MAX];
	uint32_t prog_size = PATH_MAX;

	ret = _NSGetExecutablePath(prog, &prog_size);
	if (ret) err(EX_OSERR, "_NSGetExecutablePath");

	for (i=0; i < argc; i++) {
		new_argv[i] = argv[i];
	}

	new_argv[i]   = "--switched_apptype";
	new_argv[i+1] = NULL;

	ret = posix_spawnattr_init(&attr);
	if (ret) errc(EX_OSERR, ret, "posix_spawnattr_init");

	ret = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETEXEC);
	if (ret) errc(EX_OSERR, ret, "posix_spawnattr_setflags");

	ret = posix_spawnattr_setprocesstype_np(&attr, POSIX_SPAWN_PROC_TYPE_APP_DEFAULT);
	if (ret) errc(EX_OSERR, ret, "posix_spawnattr_setprocesstype_np");

	ret = posix_spawn(NULL, prog, NULL, &attr, new_argv, environ);
	if (ret) errc(EX_OSERR, ret, "posix_spawn");
}

/*
 * Admittedly not very attractive.
 */
static void __attribute__((noreturn))
usage()
{
	errx(EX_USAGE, "Usage: zn <threads> <chain | hop | broadcast-single-sem | broadcast-per-thread> "
	     "<realtime | timeshare | fixed> <iterations> [--trace <traceworthy latency in ns>] "
	     "[--spin-one] [--spin-all] [--spin-time <nanos>] [--affinity] [--no-sleep] [--verbose]");
}

static void
parse_args(int argc, char *argv[])
{
	int ch, option_index = 0;
	char *cp;

	static struct option longopts[] = {
		{ "spin-time",          required_argument,      NULL,                           2 },
		{ "trace",              required_argument,      NULL,                           3 },
		{ "switched_apptype",   no_argument,            (int*)&g_seen_apptype,          TRUE },
		{ "spin-one",           no_argument,            (int*)&g_do_one_long_spin,      TRUE },
		{ "spin-all",           no_argument,            (int*)&g_do_all_spin,           TRUE },
		{ "affinity",           no_argument,            (int*)&g_do_affinity,           TRUE },
		{ "no-sleep",           no_argument,            (int*)&g_do_sleep,              FALSE },
		{ "verbose",            no_argument,            (int*)&g_verbose,               TRUE },
		{ "help",               no_argument,            NULL,                           'h' },
		{ NULL,                 0,                      NULL,                           0 }
	};

	while ((ch = getopt_long(argc, argv, "h", longopts, &option_index)) != -1) {
		switch (ch) {
		case 0:
			/* getopt_long set a variable */
			break;
		case 2:
			/* spin-time */
			g_do_each_spin = TRUE;
			g_each_spin_duration_ns = strtoull(optarg, &cp, 10);

			if (cp == optarg || *cp)
				errx(EX_USAGE, "arg --%s requires a decimal number, found \"%s\"",
				     longopts[option_index].name, optarg);
			break;
		case 3:
			/* trace */
			g_traceworthy_latency_ns = strtoull(optarg, &cp, 10);

			if (cp == optarg || *cp)
				errx(EX_USAGE, "arg --%s requires a decimal number, found \"%s\"",
				     longopts[option_index].name, optarg);
			break;
		case '?':
		case 'h':
		default:
			usage();
			/* NORETURN */
		}
	}

	/*
	 * getopt_long reorders all the options to the beginning of the argv array.
	 * Jump past them to the non-option arguments.
	 */

	argc -= optind;
	argv += optind;

	if (argc > 4) {
		warnx("Too many non-option arguments passed");
		usage();
	}

	if (argc != 4) {
		warnx("Missing required <threads> <waketype> <policy> <iterations> arguments");
		usage();
	}

	/* How many threads? */
	g_numthreads = (uint32_t)strtoull(argv[0], &cp, 10);

	if (cp == argv[0] || *cp)
		errx(EX_USAGE, "numthreads requires a decimal number, found \"%s\"", argv[0]);

	if (g_numthreads < 1)
		errx(EX_USAGE, "Must use at least one thread");

	/* What wakeup pattern? */
	g_waketype = parse_wakeup_pattern(argv[1]);

	/* Policy */
	g_policy = parse_thread_policy(argv[2]);

	/* Iterations */
	g_iterations = (uint32_t)strtoull(argv[3], &cp, 10);

	if (cp == argv[3] || *cp)
		errx(EX_USAGE, "numthreads requires a decimal number, found \"%s\"", argv[3]);

	if (g_iterations < 1)
		errx(EX_USAGE, "Must have at least one iteration");

	if (g_numthreads == 1 && g_waketype == WAKE_CHAIN)
		errx(EX_USAGE, "chain mode requires more than one thread");

	if (g_numthreads == 1 && g_waketype == WAKE_HOP)
		errx(EX_USAGE, "hop mode requires more than one thread");
}