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

[Stefan Hajnoczi]

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

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