Re: [Qemu-devel] is there a limit on the number of in-flight I/O operations?

[Chris Friesen]

On 08/25/2014 09:12 AM, Chris Friesen wrote:

> I set up another test, checking the inflight value every second.
>
> Running just "dd if=/dev/zero of=testfile2 bs=1M count=700
> oflag=nocache&" gave a bit over 100 inflight requests.
>
> If I simultaneously run "dd if=testfile of=/dev/null bs=1M count=700
> oflag=nocache&" then then number of inflight write requests peaks at 176.
>
> I should point out that the above numbers are with qemu 1.7.0, with a
> ceph storage backend.  qemu is started with
>
> -drive file=rbd:cinder-volumes/.........

From a stacktrace that I added it looks like the writes are coming in 
via virtio_blk_handle_output().

Looking at virtio_blk_device_init() I see it calling 
virtio_add_queue(vdev, 128, virtio_blk_handle_output);

I wondered if that 128 had anything to do with the number of inflight 
requests, so I tried recompiling with 16 instead. I still saw the number 
of inflight requests go up to 178 and the guest took a kernel panic in 
virtqueue_add_buf() so that wasn't very successful. :)

Following the code path in virtio_blk_handle_write() it looks like it 
will bundle up to 32 writes into a single large iovec-based "multiwrite" 
operation.  But from there on down I don't see a limit on how many 
writes can be outstanding at any one time.  Still checking the code 
further up the virtio call chain.

Chris