Re: [dpdk-dev] [PATCH v1 2/2] lib/vhost: restrict pointer aliasing for packed path

[Adrian Moreno <amorenoz@redhat.com>]

On 6/11/20 5:32 AM, Joyce Kong wrote:
> Restrict pointer aliasing to allow the compiler to vectorize
> loops more aggressively.
> 
> With this patch, a 9.6% improvement is observed in throughput for the
> packed virtio-net PVP case, and a 2.8% improvement in throughput for
> the packed virtio-user PVP case. All performance data are measured
> under 0.001% acceptable packet loss with 1 core on both vhost and
> virtio side.

Is the performance gain related solely to this patch or is it the result of the
combined series?

> 
> Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> Reviewed-by: Phil Yang <phil.yang@arm.com>
> ---
>  lib/librte_vhost/virtio_net.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index 751c1f373..39c92e7e1 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -1133,8 +1133,8 @@ virtio_dev_rx_single_packed(struct virtio_net *dev,
>  
>  static __rte_noinline uint32_t
>  virtio_dev_rx_packed(struct virtio_net *dev,
> -		     struct vhost_virtqueue *vq,
> -		     struct rte_mbuf **pkts,
> +		     struct vhost_virtqueue *__restrict vq,
> +		     struct rte_mbuf **__restrict pkts,
>  		     uint32_t count)
>  {
>  	uint32_t pkt_idx = 0;

I wonder if we're extracting the full potential of "restrict" considering that
the heavy lifting is done by the inner functions:
(virtio_dev_rx_batch_packed and virtio_dev_rx_single_packed)

> @@ -1219,7 +1219,7 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
>  
>  uint16_t
>  rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
> -	struct rte_mbuf **pkts, uint16_t count)
> +	struct rte_mbuf **__restrict pkts, uint16_t count)
>  {
>  	struct virtio_net *dev = get_device(vid);
>  
Is this considered an api change?

> @@ -2124,9 +2124,9 @@ free_zmbuf(struct vhost_virtqueue *vq)
>  
>  static __rte_noinline uint16_t
>  virtio_dev_tx_packed_zmbuf(struct virtio_net *dev,
> -			   struct vhost_virtqueue *vq,
> +			   struct vhost_virtqueue *__restrict vq,
>  			   struct rte_mempool *mbuf_pool,
> -			   struct rte_mbuf **pkts,
> +			   struct rte_mbuf **__restrict pkts,
>  			   uint32_t count)
>  {
>  	uint32_t pkt_idx = 0;
> @@ -2160,9 +2160,9 @@ virtio_dev_tx_packed_zmbuf(struct virtio_net *dev,
>  
>  static __rte_noinline uint16_t
>  virtio_dev_tx_packed(struct virtio_net *dev,
> -		     struct vhost_virtqueue *vq,
> +		     struct vhost_virtqueue *__restrict vq,
>  		     struct rte_mempool *mbuf_pool,
> -		     struct rte_mbuf **pkts,
> +		     struct rte_mbuf **__restrict pkts,
>  		     uint32_t count)
>  {
>  	uint32_t pkt_idx = 0;
> 

-- 
Adrián Moreno