February 10, 2002
CMUCL's Thread Hostility

I discovered another terrible bit of code I had written, where my naive expectations of what cmucl would do and the disappointing reality of cmucl's behavior conspired to destroy any semblance of interactivity in Echo.

I was using ext:run-program to run a unix executable. And I gave it a :wait argument of T because, well, I really wanted the program to finish running before I returned a value from my function. And it worked great, except that it completely blocked all lisp threads from running until the other program finished. I sigh.

The problem is in ext:process-wait, which is what ext:run-program uses when :wait is T:

(defun process-wait (proc &optional check-for-stopped)
  "Wait for PROC to quit running for some reason.  Returns PROC."
  (declare (type process proc))
    (case (process-status proc)
       (when check-for-stopped
       (when (zerop (car (process-cookie proc)))
    (system:serve-all-events 1))

(Actually, it seems to me that the call to system:serve-all-events should cause other threads to run, but experimentally I have verified that they don't.)

I basically replaced ext:process-wait with the following code, which makes everything work better:

(mp:process-wait "waiting for exec"
                 #'(lambda ()
                     (let ((status (process-status process)))
                       (and (not (eq status :running))
                            (not (eq status :signaled))))))

Well, that can't help but improve responsiveness of a server that runs external programs constantly, can it? As long as it is correct and safe. Please let it be correct and safe.

cmucl's behavior here probably doesn't count as a bug, but it sure isn't the best one could hope for. I think it was Tim Moore that once said something about trying to decide whether it would be better to rewrite cmucl to be as non-blocking as possible, or to add native thread support. I suppose the right thing to do is... both.

Posted by jjwiseman at February 10, 2002 12:50 AM
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