Re: [Qemu-devel] [PATCH] virtio-scsi: Add virtqueue_size parameter allowing virtqueue size to be set.

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Aug 10, 2017 at 05:52:55PM +0100, Richard W.M. Jones wrote:
> Since Linux switched to blk-mq as the default in Linux commit
> 5c279bd9e406 ("scsi: default to scsi-mq"), virtio-scsi LUNs consume
> about 10x as much guest kernel memory.
> 
> This commit allows you to choose the virtqueue size for each
> virtio-scsi-pci controller like this:
> 
>   -device virtio-scsi-pci,id=scsi,virtqueue_size=16
> 
> The default is still 128 as before.  Using smaller virtqueue_size
> allows many more disks to be added to small memory virtual machines.
> For a 1 vCPU, 500 MB, no swap VM I observed:
> 
>   With scsi-mq enabled (upstream kernel):              175 disks
>     -"- ditto -"-   virtqueue_size=64:                 318 disks
>     -"- ditto -"-   virtqueue_size=16:                 775 disks
>   With scsi-mq disabled (kernel before 5c279bd9e406): 1755 disks
> 
> Note that to have any effect, this requires a kernel patch:
> 
>   https://lkml.org/lkml/2017/8/10/689
> 
> Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
> ---
>  hw/scsi/virtio-scsi.c           | 8 +++++---
>  include/hw/virtio/virtio-scsi.h | 2 +-
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
> index eb639442d1..aca1909a59 100644
> --- a/hw/scsi/virtio-scsi.c
> +++ b/hw/scsi/virtio-scsi.c
> @@ -867,10 +867,10 @@ void virtio_scsi_common_realize(DeviceState *dev,
>      s->sense_size = VIRTIO_SCSI_SENSE_DEFAULT_SIZE;
>      s->cdb_size = VIRTIO_SCSI_CDB_DEFAULT_SIZE;
>  
> -    s->ctrl_vq = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, ctrl);
> -    s->event_vq = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, evt);
> +    s->ctrl_vq = virtio_add_queue(vdev, s->conf.virtqueue_size, ctrl);
> +    s->event_vq = virtio_add_queue(vdev, s->conf.virtqueue_size, evt);
>      for (i = 0; i < s->conf.num_queues; i++) {
> -        s->cmd_vqs[i] = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, cmd);
> +        s->cmd_vqs[i] = virtio_add_queue(vdev, s->conf.virtqueue_size, cmd);
>      }
>  }

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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