Timers

Real-time programs often require functions to execute at predetermined timing or to repeat in particular intervals. Sequential EusLisp could run user' functions triggered by signals generated periodically by Unix's interval timers. This preemption can cause deadlock in MT-Eus, because interruption may occur within a mutex-ed block. Therefore, control must be transferred at secured points such as at the beginning of eval. To avoid delays caused by the above synchronization, MT-Eus also provides signal-notification via semaphores. In other words, the signal function takes either a function or a semaphore that is called or posted upon the signal arrival. Since the semaphore is posted at the lowest level, latency for synchronization is minimal.

The following a example image processing program coded by using the multithread facilities. Image input thread and filtering threads are created. samp-image takes image data periodically by waiting for samp-sem to be posted every 33msec. Two threads synchronize via read-and-write of a thread-port. Filter-image employs two more threads for parallel computation of filtering.

(make-threads 8)
(defun samp-image (p)
   (let ((samp-sem (make-semaphore)))
        (periodic-sema-post 0.03 samp-sem)
        (loop (sema-wait samp-sem)
              (send p :write (read-image))))
(defun filter-image (p)
  (let (img)
       (loop (setf img (send p :read))
             (plist (filter-up-half img)
                    (filter-low-half img)))))
(setf port (make-thread-port))
(setf sampler (thread #'samp-image port))
(setf filter (thread #'filter-image port))