Re: [Qemu-devel] Following up questions related to QEMU and I/O Thread

[Wei Li]

Thanks Stefan!

Does this mean the performance could be improved via adding Batch I/O 
submission support in Guest driver side which will be able to reduce the number 
of virtqueue kicks?

Thanks,
Wei

On 4/29/19, 6:40 AM, "Stefan Hajnoczi" <address@hidden> wrote:

    On Fri, Apr 26, 2019 at 10:14:16AM +0200, Paolo Bonzini wrote:
    > On 23/04/19 14:04, Stefan Hajnoczi wrote:
    > >> In addition, does Virtio-scsi support Batch I/O Submission feature
    > >> which may be able to increase the IOPS via reducing the number of
    > >> system calls?
    > >
    > > I don't see obvious batching support in drivers/scsi/virtio_scsi.c.
    > > The Linux block layer supports batching but I'm not sure if the SCSI
    > > layer does.
    > 
    > I think he's referring to QEMU, in which case yes, virtio-scsi does
    > batch I/O submission.  See virtio_scsi_handle_cmd_req_prepare and
    > virtio_scsi_handle_cmd_req_submit in hw/scsi/virtio-scsi.c, they do
    > blk_io_plug and blk_io_unplug in order to batch I/O requests from QEMU
    > to the host kernel.
    
    This isn't fully effective since the guest driver kicks once per
    request.  Therefore QEMU-level batching you mentioned only works if QEMU
    is slower at handling virtqueue kicks than the guest is at submitting
    requests.
    
    I wonder if this is something that can be improved.
    
    Stefan