Re: [Qemu-devel] [PATCH] virtio: limit avail bytes lookahead

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 01, 2012 at 06:07:21PM +0200, Michael S. Tsirkin wrote:
> Commit 0d8d7690850eb0cf2b2b60933cf47669a6b6f18f introduced
> a regression in virtio-net performance because it looks
> into the ring aggressively while we really only care
> about a single packet worth of buffers.
> To fix, add parameters limiting lookahead, and
> use in virtqueue_avail_bytes.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>

Ping.
Anthony - going to apply this?


> ---
> 
> Edivaldo could you please confirm this fixes regression?
> 
> diff --git a/hw/virtio-serial-bus.c b/hw/virtio-serial-bus.c
> index d20bd8b..a761cdc 100644
> --- a/hw/virtio-serial-bus.c
> +++ b/hw/virtio-serial-bus.c
> @@ -297,7 +297,7 @@ size_t virtio_serial_guest_ready(VirtIOSerialPort *port)
>      if (use_multiport(port->vser) && !port->guest_connected) {
>          return 0;
>      }
> -    virtqueue_get_avail_bytes(vq, &bytes, NULL);
> +    virtqueue_get_avail_bytes(vq, &bytes, NULL, UINT_MAX, 0);
>      return bytes;
>  }
>  
> diff --git a/hw/virtio.c b/hw/virtio.c
> index ec8b7d8..f40a8c5 100644
> --- a/hw/virtio.c
> +++ b/hw/virtio.c
> @@ -336,7 +336,8 @@ static unsigned virtqueue_next_desc(hwaddr desc_pa,
>  }
>  
>  void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
> -                               unsigned int *out_bytes)
> +                               unsigned int *out_bytes,
> +                               unsigned max_in_bytes, unsigned max_out_bytes)
>  {
>      unsigned int idx;
>      unsigned int total_bufs, in_total, out_total;
> @@ -385,6 +386,9 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
>              } else {
>                  out_total += vring_desc_len(desc_pa, i);
>              }
> +            if (in_total >= max_in_bytes && out_total >= max_out_bytes) {
> +                goto done;
> +            }
>          } while ((i = virtqueue_next_desc(desc_pa, i, max)) != max);
>  
>          if (!indirect)
> @@ -392,6 +396,7 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
>          else
>              total_bufs++;
>      }
> +done:
>      if (in_bytes) {
>          *in_bytes = in_total;
>      }
> @@ -405,12 +410,8 @@ int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
>  {
>      unsigned int in_total, out_total;
>  
> -    virtqueue_get_avail_bytes(vq, &in_total, &out_total);
> -    if ((in_bytes && in_bytes < in_total)
> -        || (out_bytes && out_bytes < out_total)) {
> -        return 1;
> -    }
> -    return 0;
> +    virtqueue_get_avail_bytes(vq, &in_total, &out_total, in_bytes, out_bytes);
> +    return in_bytes <= in_total && out_bytes <= out_total;
>  }
>  
>  void virtqueue_map_sg(struct iovec *sg, hwaddr *addr,
> diff --git a/hw/virtio.h b/hw/virtio.h
> index ac482be..1278328 100644
> --- a/hw/virtio.h
> +++ b/hw/virtio.h
> @@ -150,7 +150,8 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem);
>  int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
>                            unsigned int out_bytes);
>  void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
> -                               unsigned int *out_bytes);
> +                               unsigned int *out_bytes,
> +                               unsigned max_in_bytes, unsigned max_out_bytes);
>  
>  void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);
>