Hi,
> In the slow path of a spinlock_acquire they busy wait for a few
> cycles, and then call schedule with a zero timeout assuming that
> it'll basically do the same as a sched_yield() but more portably.
The obvious problem with this is that we bounce in and out of schedule()
a few times before moving on to the next task. I see this also with
sched_yield().
I had this patch lying around which I think came about when I was playing
with pthreads (which for spinlocks does sched_yield() for a while before
sleeping)
--- linux/kernel/sched.c Fri Mar 9 10:26:56 2001
+++ linux_intel/kernel/sched.c Fri Mar 9 08:42:39 2001
@@ -505,6 +505,9 @@
goto out_unlock;
}
#else
+ if (prev->policy & SCHED_YIELD)
+ prev->counter = (prev->counter >> 4);
+
prev->policy &= ~SCHED_YIELD;
#endif /* CONFIG_SMP */
}
Anton
/* test sched_yield */
#include <stdio.h>
#include <sched.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#undef USE_SELECT
void waste_time()
{
int i;
for(i = 0; i < 10000; i++)
;
}
void do_stuff(int i)
{
#ifdef USE_SELECT
struct timeval tv;
#endif
while(1) {
fprintf(stderr, "%d\n", i);
waste_time();
#ifdef USE_SELECT
tv.tv_sec = 0;
tv.tv_usec = 0;
select(0, NULL, NULL, NULL, &tv);
#else
sched_yield();
#endif
}
}
int main()
{
int i, pid;
for(i = 0; i < 10; i++) {
pid = fork();
if (!pid)
do_stuff(i);
}
do_stuff(i+1);
return 0;
}
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