I have two simple microbenchmarks trying to measure thread- and process-switching overheads, but the process-switching overhead is turning out to be lower than that of thread-switching, which is unexpected. The setup: 1.8GHz Core 2 Duo, 2GB RAM, Linux 2.6.32-21-generic x86_64 (Ubuntu 10.04). I'm getting:
- ~2.1-2.4us per process switch
- ~4us per thread switch
I tried also running with numactl --physcpubind=0 and likwid-pin -c0, but this seemed to only slow down the thread switches to 5us. Anybody know what's wrong with the evaluation, or if these results are right why they are?
The code is living at the URLs below, and r1667 is pasted here:
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;
}