qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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


From: Paolo Bonzini
Subject: Re: [Qemu-devel] Re: [PATCH V4 2/3] qemu: Generic task offloading framework: threadlets
Date: Wed, 16 Jun 2010 16:52:27 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100430 Fedora/3.0.4-3.fc13 Lightning/1.0b2pre Thunderbird/3.0.4

On 06/16/2010 04:38 PM, Anthony Liguori wrote:
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)...

Agreed---no cancellation, please.

BTW it's obviously okay with signaling the condition when a threadlet is submitted. But when something affects all queue's workers (flush_threadlet_queue) you want a broadcast and using expiration as a substitute is fishy.

+ 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.

Yes, of course.  The correct test should be:

  if (queue->exit || queue->cur_threads > queue->min_threads)

But queue->idle_threads will be > 0 even if coming via the goto (which should be eliminated).

Or maybe no. After flushing you still want min_threads threads to run. The correct thing then would be:

    do {
        ...
        assert (queue->idle_threads > 0);
        if (queue->exit) {
            /* Threads waiting on the barrier cannot do work.  */
            queue->idle_threads--;
            qemu_mutex_unlock(&(queue->lock));
            qemu_barrier_wait(&queue->barr);
            qemu_mutex_lock(&(queue->lock));
            queue->idle_threads++;
        }
    } while (queue->cur_threads <= queue->min_threads);

    queue->idle_threads--;
    queue->cur_threads--;
    qemu_mutex_unlock(&queue->lock);
    return NULL;

So, if min_threads were changed, broadcasting the condition would be enough to exit unwanted threads one at a time, as soon as it grabs the lock.

Paolo



reply via email to

[Prev in Thread] Current Thread [Next in Thread]