sched_setaffinity(), RT priorities and migration thread usage at30%
From: don fisher
Date: Mon Sep 06 2004 - 17:46:26 EST
Hello,
Apologies in advance if this is a newbie question. I am attempting to
write a real-time simulation of an application we have in house. I
have a dual processor SMP system, hyperthreading enabled, running
kernel 2.6.7.
The first thread begins at priority 1 (SCHED_RR) and subsequently
spawns another time critical task running at priority 2. The initial
thread uses setaffinity to set the desired cpu to 2. When the second
task begins, the migration thread becomes 30% active (as reported by
top) for the duration of its execution. When the priority 2 thread
terminates the first thread continues with the migration task
consuming only 2% of the CPU.
If there was any change, I was expecting that the higher priority of
the second thread would cause it to execute closer to 100% CPU. I
built a test code where each thread computes an identical dumb timing
loop. The priority 2 thread ends up executing 30% slower than the
priority 1 thread due to contention with the migration thread.
Is this the expected behavior, and if so could you please inform me
why? I had not anticipated the any attempt by the kernel to shift the
process to another CPU, since sched_setafinity had been applied.
I want to confirm that from xosview, both tasks do actually execute
serially on processor 2 as expected. Code is include below.
don
int main(argc, argv)
{
cpu_set_t cur_mask;
struct sched_param policy;
long ret;
u32 page_size;
pthread_attr_t attr;
pthread_t task_pid;
CPU_ZERO(&cur_mask);
CPU_SET(2, &cur_mask);
ret = sched_setaffinity(0, sizeof(cur_mask), &cur_mask);
policy.sched_priority = 1;
ret = sched_setparam(0, &policy);
ret = sched_setscheduler(0, SCHED_RR, &policy);
ret = pthread_attr_init(&attr);
ret = pthread_attr_setschedpolicy(&attr, SCHED_RR);
policy.sched_priority = 2;
ret = pthread_attr_setschedparam(&attr, &policy);
ret = pthread_attr_getstacksize(&attr, &stack_size);
ret = pthread_create(&task_pid, &attr, (void *(*)())
local_thread, (void *)NULL);
{
long i, j = 0;
fprintf(stderr, "enter %ld\n", j);
for(i = 0; i < 1000000000; i++){
j += sqrt((double)i);
}
fprintf(stderr, "end %ld\n", j);
}
policy.sched_priority = 0;
ret = sched_setparam(0, &policy);
ret = sched_setscheduler(0, SCHED_OTHER, &policy);
return 0;
}
int32 local_thread(void)
{
{
long i, j = 0;
fprintf(stderr, "thread %ld\n", j);
for(i = 0; i < 1000000000; i++){
j += sqrt((double)i);
}
fprintf(stderr, "thread %ld\n", j);
}
return 0;
}
-
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