Re: [Qemu-devel] [RFC] block-queue: Delay and batch metadata writes

[Kevin Wolf]

Am 20.09.2010 16:56, schrieb Anthony Liguori:
>>> +void blkqueue_flush(BlockQueue *bq)
>>> +{
>>> +    qemu_mutex_lock(&bq->flush_lock);
>>> +
>>> +    /* Process any left over requests */
>>> +    while (QTAILQ_FIRST(&bq->queue)) {
>>> +        blkqueue_process_request(bq);
>>> +    }
>>> +
>>> +    qemu_mutex_unlock(&bq->flush_lock);
>>> +}
>>> +
>>> +static void *blkqueue_thread(void *_bq)
>>> +{
>>> +    BlockQueue *bq = _bq;
>>> +#ifndef RUN_TESTS
>>> +    BlockQueueRequest *req;
>>> +#endif
>>> +
>>> +    qemu_mutex_lock(&bq->flush_lock);
>>> +    while (!bq->thread_done) {
>>> +        barrier();
> 
> A barrier shouldn't be needed here.

It was needed when I started with an empty thread because gcc would
"optimize" while(!bq->thread_done) into an endless loop. I guess there
is enough code added now that gcc won't try to be clever any more, so I
can remove that.

>>> +#ifndef RUN_TESTS
>>> +        req = QTAILQ_FIRST(&bq->queue);
>>> +
>>> +        /* Don't process barriers, we only do that on flushes */
>>> +        if (req&&  (req->type != REQ_TYPE_BARRIER || 
>>> bq->queue_size>  42)) {
>>> +            blkqueue_process_request(bq);
>>> +        } else {
>>> +            qemu_cond_wait(&bq->cond,&bq->flush_lock);
>>> +        }
> 
> 
> The normal pattern for this is:
> 
> while (!condition) {
>      qemu_cond_wait(&cond, &lock);
> }
> process_request()
> 
> It's generally best not to deviate from this pattern in terms of code 
> readability.

Hm, yes, I think you're right. The code used to be a bit more involved
here initially and it seems that I missed the last obvious piece of
simplification.

> A less invasive way of doing this (assuming we're okay with it from a 
> correctness perspective) is to make use of qemu_aio_wait() as a 
> replacement for qemu_mutex_lock() and shift the pread/pwrite calls to 
> bdrv_aio_write/bdrv_aio_read.
> 
> IOW, blkqueue_pwrite stages a request via bdrv_aio_write().  
> blkqueue_pread() either returns a cached read or it does a 
> bdrv_pread().  The blkqueue_flush() call will then do qemu_aio_wait() to 
> wait for all pending I/Os to complete.

I was actually considering that, but it would have been a bit more
coding to keep track of another queue of in-flight requests, juggling
with some more AIOCBs and implementing an emulation for the missing
bdrv_aio_pwrite. Nothing really dramatic, it just was easier to start
this way.

If we come to the conclusion that bdrv_aio_write is the way to go and
it's worth the work, I'm fine with changing it.

Kevin