Ingo Molnar a écrit :
* Jie Chen <[email protected]> wrote:
I just ran the same test on two 2.6.24-rc4 kernels: one with
CONFIG_FAIR_GROUP_SCHED on and the other with CONFIG_FAIR_GROUP_SCHED
off. The odd behavior I described in my previous e-mails were still
there for both kernels. Let me know If I can be any more help. Thank
you.
ok, i had a look at your data, and i think this is the result of the
scheduler balancing out to idle CPUs more agressively than before. Doing
that is almost always a good idea though - but indeed it can result in
"bad" numbers if all you do is to measure the ping-pong "performance"
between two threads. (with no real work done by any of them).
the moment you saturate the system a bit more, the numbers should
improve even with such a ping-pong test.
do you have testcode (or a modification of your testcase sourcecode)
that simulates a real-life situation where 2.6.24-rc4 performs not as
well as you'd like it to see? (or if qmt.tar.gz already contains that
then please point me towards that portion of the test and how i should
run it - thanks!)
Ingo
I cooked a program shorter than Jie one, to try to understand what was going
on. Its a pure cpu burner program, with no thread synchronisation (but the
pthread_join at the very end)
As each thread is bound to a given cpu, I am not sure the scheduler is allowed
to balance to an idle cpu.
Unfortunatly I dont have a 4 way SMP idle machine available to
test it.
$ gcc -O2 -o burner burner.c
$ ./burner
Time to perform the unit of work on one thread is 0.040328 s
Time to perform the unit of work on 2 threads is 0.040221 s
I tried it on a 64 way machine (Thanks David :) ) and noticed some strange
results that may be related to the Niagara hardware (time for 64 threads was
nearly the double for one thread)
#include <pthread.h>
#include <sched.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/time.h>
#include <stdio.h>
#include <stdlib.h>
int blockthemall=1;
static void inline cpupause()
{
#if defined(i386)
asm volatile("rep;nop":::"memory");
#else
asm volatile("":::"memory");
#endif
}
/*
* Determines number of cpus
* Can be overiden by the NR_CPUS environment variable
*/
int number_of_cpus()
{
char line[1024], *p;
int cnt = 0;
FILE *F;
p = getenv("NR_CPUS");
if (p)
return atoi(p);
F = fopen("/proc/cpuinfo", "r");
if (F == NULL) {
perror("/proc/cpuinfo");
return 1;
}
while (fgets(line, sizeof(line), F) != NULL) {
if (memcmp(line, "processor", 9) == 0)
cnt++;
}
fclose(F);
return cnt;
}
void compute_elapsed(struct timeval *delta, const struct timeval *t0)
{
struct timeval t1;
gettimeofday(&t1, NULL);
delta->tv_sec = t1.tv_sec - t0->tv_sec;
delta->tv_usec = t1.tv_usec - t0->tv_usec;
if (delta->tv_usec < 0) {
delta->tv_usec += 1000000;
delta->tv_sec--;
}
}
int nr_loops = 20*1000000;
double incr=0.3456;
void perform_work()
{
int i;
double t = 0.0;
for (i = 0; i < nr_loops; i++) {
t += incr;
}
if (t < 0.0) printf("well... should not happen\n");
}
void set_affinity(int cpu)
{
long cpu_mask;
int res;
cpu_mask = 1L << cpu;
res = sched_setaffinity(0, sizeof(cpu_mask), &cpu_mask);
if (res)
perror("sched_setaffinity");
}
void *thread_work(void *arg)
{
int cpu = (int)arg;
set_affinity(cpu);
while (blockthemall)
cpupause();
perform_work();
return (void *)0;
}
main(int argc, char *argv[])
{
struct timeval t0, delta;
int nr_cpus, i;
pthread_t *tids;
gettimeofday(&t0, NULL);
perform_work();
compute_elapsed(&delta, &t0);
printf("Time to perform the unit of work on one thread is %d.%06d s\n", delta.tv_sec, delta.tv_usec);
nr_cpus = number_of_cpus();
if (nr_cpus <= 1)
return 0;
tids = malloc(nr_cpus * sizeof(pthread_t));
for (i = 1; i < nr_cpus; i++) {
pthread_create(tids + i, NULL, thread_work, (void *)i);
}
set_affinity(0);
gettimeofday(&t0, NULL);
blockthemall=0;
perform_work();
for (i = 1; i < nr_cpus; i++)
pthread_join(tids[i], NULL);
compute_elapsed(&delta, &t0);
printf("Time to perform the unit of work on %d threads is %d.%06d s\n", nr_cpus, delta.tv_sec, delta.tv_usec);
}
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