Microbenchmark showing process-switching faster than thread-switching; what's wrong?
- by Yang
I have two simple microbenchmarks trying to measure thread- and process-switching overheads, but the process-switching overhead.
The code is living here, and r1667 is pasted below:
https://assorted.svn.sourceforge.net/svnroot/assorted/sandbox/trunk/src/c/process_switch_bench.c
// on zs, ~2.1-2.4us/switch
#include <stdlib.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <semaphore.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/time.h>
#include <pthread.h>
uint32_t COUNTER;
pthread_mutex_t LOCK;
pthread_mutex_t START;
sem_t *s0, *s1, *s2;
void * threads (
void * unused
) {
// Wait till we may fire away
sem_wait(s2);
for (;;) {
pthread_mutex_lock(&LOCK);
pthread_mutex_unlock(&LOCK);
COUNTER++;
sem_post(s0);
sem_wait(s1);
}
return 0;
}
int64_t timeInMS ()
{
struct timeval t;
gettimeofday(&t, NULL);
return (
(int64_t)t.tv_sec * 1000 +
(int64_t)t.tv_usec / 1000
);
}
int main (
int argc,
char ** argv
) {
int64_t start;
pthread_t t1;
pthread_mutex_init(&LOCK, NULL);
COUNTER = 0;
s0 = sem_open("/s0", O_CREAT, 0022, 0);
if (s0 == 0) { perror("sem_open"); exit(1); }
s1 = sem_open("/s1", O_CREAT, 0022, 0);
if (s1 == 0) { perror("sem_open"); exit(1); }
s2 = sem_open("/s2", O_CREAT, 0022, 0);
if (s2 == 0) { perror("sem_open"); exit(1); }
int x, y, z;
sem_getvalue(s0, &x);
sem_getvalue(s1, &y);
sem_getvalue(s2, &z);
printf("%d %d %d\n", x, y, z);
pid_t pid = fork();
if (pid) {
pthread_create(&t1, NULL, threads, NULL);
pthread_detach(t1);
// Get start time and fire away
start = timeInMS();
sem_post(s2);
sem_post(s2);
// Wait for about a second
sleep(1);
// Stop thread
pthread_mutex_lock(&LOCK);
// Find out how much time has really passed. sleep won't guarantee me that
// I sleep exactly one second, I might sleep longer since even after being
// woken up, it can take some time before I gain back CPU time. Further
// some more time might have passed before I obtained the lock!
int64_t time = timeInMS() - start;
// Correct the number of thread switches accordingly
COUNTER = (uint32_t)(((uint64_t)COUNTER * 2 * 1000) / time);
printf("Number of process switches in about one second was %u\n", COUNTER);
printf("roughly %f microseconds per switch\n", 1000000.0 / COUNTER);
// clean up
kill(pid, 9);
wait(0);
sem_close(s0);
sem_close(s1);
sem_unlink("/s0");
sem_unlink("/s1");
sem_unlink("/s2");
} else {
if (1) { sem_t *t = s0; s0 = s1; s1 = t; }
threads(0); // never return
}
return 0;
}
https://assorted.svn.sourceforge.net/svnroot/assorted/sandbox/trunk/src/c/thread_switch_bench.c
// From <http://stackoverflow.com/questions/304752/how-to-estimate-the-thread-context-switching-overhead>
// on zs, ~4-5us/switch; tried making COUNTER updated only by one thread, but no difference
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/time.h>
uint32_t COUNTER;
pthread_mutex_t LOCK;
pthread_mutex_t START;
pthread_cond_t CONDITION;
void * threads (
void * unused
) {
// Wait till we may fire away
pthread_mutex_lock(&START);
pthread_mutex_unlock(&START);
int first=1;
pthread_mutex_lock(&LOCK);
// If I'm not the first thread, the other thread is already waiting on
// the condition, thus Ihave to wake it up first, otherwise we'll deadlock
if (COUNTER > 0) {
pthread_cond_signal(&CONDITION);
first=0;
}
for (;;) {
if (first) COUNTER++;
pthread_cond_wait(&CONDITION, &LOCK);
// Always wake up the other thread before processing. The other
// thread will not be able to do anything as long as I don't go
// back to sleep first.
pthread_cond_signal(&CONDITION);
}
pthread_mutex_unlock(&LOCK);
return 0;
}
int64_t timeInMS ()
{
struct timeval t;
gettimeofday(&t, NULL);
return (
(int64_t)t.tv_sec * 1000 +
(int64_t)t.tv_usec / 1000
);
}
int main (
int argc,
char ** argv
) {
int64_t start;
pthread_t t1;
pthread_t t2;
pthread_mutex_init(&LOCK, NULL);
pthread_mutex_init(&START, NULL);
pthread_cond_init(&CONDITION, NULL);
pthread_mutex_lock(&START);
COUNTER = 0;
pthread_create(&t1, NULL, threads, NULL);
pthread_create(&t2, NULL, threads, NULL);
pthread_detach(t1);
pthread_detach(t2);
// Get start time and fire away
start = timeInMS();
pthread_mutex_unlock(&START);
// Wait for about a second
sleep(1);
// Stop both threads
pthread_mutex_lock(&LOCK);
// Find out how much time has really passed. sleep won't guarantee me that
// I sleep exactly one second, I might sleep longer since even after being
// woken up, it can take some time before I gain back CPU time. Further
// some more time might have passed before I obtained the lock!
int64_t time = timeInMS() - start;
// Correct the number of thread switches accordingly
COUNTER = (uint32_t)(((uint64_t)COUNTER * 2 * 1000) / time);
printf("Number of thread switches in about one second was %u\n", COUNTER);
printf("roughly %f microseconds per switch\n", 1000000.0 / COUNTER);
return 0;
}
