* Stefan Hajnoczi <stefa...@gmail.com> [2010-11-10 13:45:29]:
> On Wed, Nov 10, 2010 at 1:19 PM, Arun R Bharadwaj
> <a...@linux.vnet.ibm.com> wrote:
> > @@ -301,106 +431,58 @@ static ssize_t handle_aiocb_rw(struct qemu_paiocb
> > *aiocb)
> > return nbytes;
> > }
> >
> > -static void *aio_thread(void *unused)
> > +static void aio_thread(ThreadletWork *work)
> > {
> > pid_t pid;
> > + struct qemu_paiocb *aiocb = container_of(work, struct qemu_paiocb,
> > work);
> > + ssize_t ret = 0;
> >
> > pid = getpid();
> >
> > - while (1) {
> > - struct qemu_paiocb *aiocb;
> > - ssize_t ret = 0;
> > - qemu_timeval tv;
> > - struct timespec ts;
> > -
> > - qemu_gettimeofday(&tv);
> > - ts.tv_sec = tv.tv_sec + 10;
> > - ts.tv_nsec = 0;
> > -
> > - mutex_lock(&lock);
> > -
> > - while (QTAILQ_EMPTY(&request_list) &&
> > - !(ret == ETIMEDOUT)) {
> > - ret = cond_timedwait(&cond, &lock, &ts);
> > - }
> > -
> > - if (QTAILQ_EMPTY(&request_list))
> > - break;
> > -
> > - aiocb = QTAILQ_FIRST(&request_list);
> > - QTAILQ_REMOVE(&request_list, aiocb, node);
> > - aiocb->active = 1;
> > - idle_threads--;
> > - mutex_unlock(&lock);
> > -
> > - switch (aiocb->aio_type & QEMU_AIO_TYPE_MASK) {
> > - case QEMU_AIO_READ:
> > - case QEMU_AIO_WRITE:
> > - ret = handle_aiocb_rw(aiocb);
> > - break;
> > - case QEMU_AIO_FLUSH:
> > - ret = handle_aiocb_flush(aiocb);
> > - break;
> > - case QEMU_AIO_IOCTL:
> > - ret = handle_aiocb_ioctl(aiocb);
> > - break;
> > - default:
> > - fprintf(stderr, "invalid aio request (0x%x)\n",
> > aiocb->aio_type);
> > - ret = -EINVAL;
> > - break;
> > - }
> > -
> > - mutex_lock(&lock);
> > - aiocb->ret = ret;
> > - idle_threads++;
> > - mutex_unlock(&lock);
> > -
> > - if (kill(pid, aiocb->ev_signo)) die("kill failed");
> > + switch (aiocb->aio_type & QEMU_AIO_TYPE_MASK) {
> > + case QEMU_AIO_READ:
> > + case QEMU_AIO_WRITE:
> > + ret = handle_aiocb_rw(aiocb);
> > + break;
> > + case QEMU_AIO_FLUSH:
> > + ret = handle_aiocb_flush(aiocb);
> > + break;
> > + case QEMU_AIO_IOCTL:
> > + ret = handle_aiocb_ioctl(aiocb);
> > + break;
> > + default:
> > + fprintf(stderr, "invalid aio request (0x%x)\n", aiocb->aio_type);
> > + ret = -EINVAL;
> > + break;
> > }
> >
> > - idle_threads--;
> > - cur_threads--;
> > - mutex_unlock(&lock);
> > -
> > - return NULL;
> > -}
> > -
> > -static void spawn_thread(void)
> > -{
> > - sigset_t set, oldset;
> > -
> > - cur_threads++;
> > - idle_threads++;
> > + qemu_mutex_lock(&aiocb_mutex);
> > + aiocb->ret = ret;
>
> This is where qemu_cond_broadcast() is needed.
>
> > + qemu_mutex_unlock(&aiocb_mutex);
> >
> > - /* block all signals */
> > - if (sigfillset(&set)) die("sigfillset");
> > - if (sigprocmask(SIG_SETMASK, &set, &oldset)) die("sigprocmask");
> > -
> > - thread_create(&thread_id, &attr, aio_thread, NULL);
> > -
> > - if (sigprocmask(SIG_SETMASK, &oldset, NULL)) die("sigprocmask
> > restore");
> > + if (kill(pid, aiocb->ev_signo)) {
> > + die("kill failed");
> > + }
> > }
> >
>
> I wonder if the condition variable has a measurable performance
> overhead. We unconditionally broadcast on paiocb completion. One
> idea would be to keep a counter of waiters (should only ever be 0 or
> 1) protected by aiocb_mutex and broadcast only when there is a waiter.
> I just want to share this idea, I don't know if it's necessary to
> implement it or if it could even work without a race condition.
>
I did not understand exactly why we are going to see a performane hit.
We will be doing a broadcast only after the aio_thread has finished
the work right? So how is this going to affect performance even if we
do a useless broadcast?
-arun
> > }
> >
> > paio_remove(acb);
> > @@ -618,11 +692,12 @@ int paio_init(void)
> > struct sigaction act;
> > PosixAioState *s;
> > int fds[2];
> > - int ret;
> >
> > if (posix_aio_state)
> > return 0;
> >
> > + qemu_mutex_init(&aiocb_mutex);
>
> Also needs qemu_cond_init(&aiocb_completion).
>
> Stefan
>
