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

[Anthony Liguori]

On 06/16/2010 09:29 AM, Paolo Bonzini wrote:
> 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

If a threadlet is waiting to consume more work, unless we do a 
pthread_cancel (I dislike cancellation) it will keep waiting until it 
gets more work (which would mean it's not actually idle)...

> +        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").

queue->exit could be true though so it's necessary to at least check 
that condition.  I agree though that via the normal fall through path, 
queue->idle_threads can never be non-zero.

Regards,

Anthony Liguori

> 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. :)

I think the code has been modified such that you are correct.

> Paolo