Lies, Misdirection, and Real-time 
by Cort Dougan and Zwane Mwaikambo


Listing 1: RTLinux Clock Resolution Report

Clock Resolution
271.26 ns, 3 MHz

Listing 2(a): RTLinux context switch time pseudocode.

Thread A
  clock_gettime(CLOCK_REALTIME, &start);
  pthread_wakeup_np(thread_B);
Thread B
  clock_gettime(CLOCK_REALTIME, &end);
  pthread_suspend_np(thread_B);

Listing 2(b): RTLinux Context Switch Time Measurement Output

Context Switch - Realtime thread to Realtime thread: (96h 0m 0s test)
48.2 us

Listing 3(a): RTLinux Interrupt Latency Pseudo-Code

ISR
   if (start) {
   ticks = rtl_getticks() - start;
   if (ticks > worst)
      worst = ticks;
   }
   start = rtl_getticks();
   rtl_settimer(period);
main()
   ....
   intr_latency = ((worst period) * NSECS_PER_SEC) / rtl_clock_tick_rate;


Listing 3(b): RTLinux Interrupt Latency Output

Interrupt Latency: (96h 0m 0s test)
39.6 us

Listing 4(a): RTLinux Jitter Test

timespec_add_ns(&ts_next_wakeup, PERIOD);
clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &ts_next_wakeup, NULL);
clock_gettime(CLOCK_REALTIME, &ts_wakeup);


Listing 4(b): RTLinux Jitter Output

Scheduling Jitter: (96h 0m 0s test)
88.2 us

Listing 5(a): RTLinux Interrupt Latency Dispatch Test

ISR
   start = rtl_getticks();
   sem_post(&semaphore);
Thread
   sem_wait(&semaphore);
   ticks = rtl_getticks() - start;


Listing 5(b): RTLinux Interrupt Latency Dispatch Output

Interrupt Thread Latency - Realtime ISR to Realtime thread (via Semaphore): (46.0 us


Listing 6(a): RTLinux Yield Test

Thread A
   clock_gettime(CLOCK_REALTIME, &start);
   sched\_yield();
   sem_wait(&semaphore);
Thread B
   clock_gettime(CLOCK_REALTIME, &end);
   sem_post(&sem);

Listing 6(b): RTLinux Yield Output

Thread Yield - Realtime thread to Realtime thread: (96h 0m 0s test)
24.9 us

Listing 7(a): RTLinux Cancel Test

main()
   for (i = 0; i < NUM_THREADS; i++)
      pthread_create(&threads[i], NULL, test_thread, NULL);
Thread
   for (i = 0; i < NUM_THREADS; i++) {
      clock_gettime(CLOCK_REALTIME, &start);
      pthread_cancel(threads[i]);
      pthread_join(threads[i], NULL);
      clock_gettime(CLOCK_REALTIME, &end);
   }


Listing 7(b): RTLinux Cancel Output

Thread Cancellation:
10051.5 us

Listing 8(a): RTLinux Semaphore Test

Thread A
   clock_gettime(CLOCK_REALTIME, &start);
   sem_post(&sem);
Thread B
   sem_wait(&sem);
   clock_gettime(CLOCK_REALTIME, &end);


Listing 8(b): RTLinux Semaphore Output

Semaphore Latency: (96h 0m 0s test)
48.0 us


Listing 9(a): RTLinux Mutex Test

Thread
   clock_gettime(CLOCK_REALTIME, &start);
   pthread_mutex_unlock(&mutex);
   pthread_mutex_lock(&mutex);
   clock_gettime(CLOCK_REALTIME, &end);


Listing 9(b): RTLinux Mutex Output

Uncontested Mutex Acquisition: (96h 0m 0s test)
19.2 us


Listing 10(a): RTLinux Contested Mutex Test

Thread A
   clock_gettime(CLOCK_REALTIME, &start);
   pthread_mutex_lock(&mutex);
   sem_post(&start_sem);
   pthread_mutex_unlock(&mutex);
Thread B
   sem_wait(&start_sem);
   pthread_mutex_lock(&mutex);
   clock_gettime(CLOCK_REALTIME, &end);
   pthread_mutex_unlock(&mutex);


Listing 10(b): RTLinux Contested Mutex Output

Contested Mutex Acquisition: (96h 0m 0s test)
40.6 us


Listing 11(a): RTLinux Priority Inversion Test

Thread A
   pthread_suspend_np(thread_A);
   clock_gettime(CLOCK_REALTIME, &start);
   pthread_mutex_lock(&mutex);
   clock_gettime(CLOCK_REALTIME, &end);
Thread B
   pthread_mutex_lock(&mutex);
   pthread_cond_wait(&cond, &mutex);
   pthread_wakeup_np(thread_A);
   do_something(SLEEP_PERIOD);
   pthread_mutex_unlock(&mutex);
Thread C
   pthread_cond_signal(&cond);


Listing 11(b): RTLinux Priority Inversion Output

Priority Inversion Recovery: (96h 0m 0s test)
64.0 us


Listing 12(a): RTLinux Conditional Variable Test

Thread A
   clock_gettime(CLOCK_REALTIME, &start);
   pthread_cond_signal(&cond);
Thread B
   pthread_mutex_lock(&cond_mutex);
   pthread_cond_wait(&cond, &cond_mutex);
   clock_gettime(CLOCK_REALTIME, &end);
   pthread_mutex_unlock(&cond_mutex);


Listing 12(b): RTLinux Conditional Variable Output

Pthread Condition Variable Latency: (96h 0m 0s test)
48.0 us


Listing 13(a): RTLinux Spinlock Test

Thread
   clock_gettime(CLOCK_REALTIME, &start);
   pthread_spin_unlock(&lock);
   pthread_spin_lock(&lock);
   clock_gettime(CLOCK_REALTIME, &end);


Listing 13(b): RTLinux Spinlock Output

Uncontested Spinlock Acquisition: (96h 0m 0s test)
16.2 us


Listing 14(a): RTLinux Contested Spinlock Test

Thread A
   pthread_spin_lock(&lock);
   sem_post(&sem);
   clock_gettime(CLOCK_REALTIME, &start);
   pthread_spin_unlock(&lock);
Thread B
   sem_wait(&start_sem);
   pthread_spin_lock(&lock);
   clock_gettime(CLOCK_REALTIME, &end);
   pthread_spin_unlock(&lock);


Listing 14(b): RTLinux Contested Spinlock Output

Contested Spinlock Acquisition: (96h 0m 0s test)
38.5 us

Listing 15(a): RTLinux FIFO test


rtl_no_interrupts(flags);
clock_gettime(clock_REALTIME, &start);
rtf_put(rtl_fds[fd].f_priv, buf, size);
clock_gettime( clock_REALTIME, &end);

clock_gettime(clock_REALTIME, &start);
for (j = 0; j < num_runs; j++)
   rtl_write(fd, buf, size);
clock_gettime(clock_REALTIME, &end);
rtl_restore_interrupts(flags);


Listing 15(b): RTLinux FIFO Output

FIFO Latency:
15 NULL rtl_write() took 22.7 us
15 NULL rtl_write() took 17.6 us
15 NULL rtl_write() took 17.3 us
15 NULL rtf_put() took 8.6 us
15 NULL rtf_put() took 8.4 us
15 NULL rtf_put() took 8.6 us

15 128b rtl_write() took 56.6 us
15 128b rtl_write() took 31.7 us
15 128b rtl_write() took 32.5 us
15 128b rtf_put() took 23.8 us
15 128b rtf_put() took 10.0 us
15 128b rtf_put() took 5.1 us


Listing 16(a): RTLinux FIFO Latency


Thread
   rtl_no_interrupts(flags);
   clock_gettime(CLOCK_REALTIME, &start);
   rtl_write(fd, buf, 0);
   rtl_restore_interrupts(flags);
Handler
   clock_gettime(CLOCK_REALTIME, &end);


Listing 16(b): RTLinux FIFO Latency Output

Testing handler latency...
NULL rtl_write() handler took 8.1 us
NULL rtl_write() handler took 2.9 us
NULL rtl_write() handler took 2.9 us



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