sched_yield() is very slow when other threads are busy on FreeBSD-current

Hi,

I experienced sched_yield() is very slow when other threads are busy
on FreeBSD-current.

When I run the program:
------------------------------------------------
#include <assert.h>
#include <stdio.h>
#include <pthread.h>

#define NUM_THREAD (10)

volatile int run = 0;

void *start(void *p)
{
while (!run)
;
return p;
}

int main()
{
pthread_t th[NUM_THREAD];
int i;
for (i = 0; i < NUM_THREAD; i++) {
assert(pthread_create(&th[i], NULL, start, NULL) == 0);
sched_yield();
}
run = 1;
for (i = 0; i < NUM_THREAD; i++)
assert(pthread_join(th[i], NULL) == 0);
return 0;
}
------------------------------------------------
The output of /usr/bin/time:

FreeBSD 6.2-RELEASE, libthr, HZ=100, GENERIC kernel:
0.55 real 0.55 user 0.00 sys

FreeBSD 7-CURRENT, libthr, HZ=100, kernel
include GENERIC
nooptions INVARIANTS, INVARIANT_SUPPORT, WITNESS, WITNESS_SKIPSPIN:
164.16 real 162.80 user 0.00 sys

I tried SCHED_ULE too, but does not help so much.

Increasing HZ helps to perform better, but still very slower than FreeBSD-6.

Thanks,
--
NIIMI Satoshi
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