Re: [Qemu-devel] Re: [PATCH V4 2/3] qemu: Generic task offloading framework: threadlets

[Paolo Bonzini]

On 06/16/2010 04:22 PM, Jamie Lokier wrote:
> Paolo Bonzini wrote:
> > These should be (at least for now) block-obj-$(CONFIG_POSIX).
> >
> > > +        while (QTAILQ_EMPTY(&(queue->request_list))&&
> > > +               (ret != ETIMEDOUT)) {
> > > +            ret = qemu_cond_timedwait(&(queue->cond),
> > > +                                       &(queue->lock), 10*100000);
> > > +        }
> >
> > Using qemu_cond_timedwait is a hack for not properly broadcasting the
> > condvar in flush_threadlet_queue.
>
> Are you sure?  It looks like it also expires idle threads after a
> fixed amount of idle time.

Unnecessary idle threads are immediately expired as soon as the 
threadlet exits if ncecessary, since here

+        queue->idle_threads++;
+
+check_exit:
+        if (queue->exit || ((queue->idle_threads > 0) &&
+            (queue->cur_threads > queue->min_threads))) {
+            /* We exit the queue or we retain minimum number of threads */
+            break;
+        }

queue->idle_threads > 0 will always be true (so maybe that should be 
changed into an assertion: "this thread is idle, so there must be idle 
threads").

The min/max_threads parameters of the queue are currently immutable, so 
it can never happen that a thread has to be expired while it's waiting. 
 It may well become true in the future, in which case the condvar will 
have to be broadcast when min_threads changes.

I may well be wrong of course. :)

Paolo