Re: [dpdk-dev] [PATCH v1 1/2] net/virtio: restrict pointer aliasing for NEON vpmd

[Phil Yang <Phil.Yang@arm.com>]

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: Tuesday, June 23, 2020 4:48 PM
> To: Joyce Kong <Joyce.Kong@arm.com>; jerinj@marvell.com;
> zhihong.wang@intel.com; xiaolong.ye@intel.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Phil Yang <Phil.Yang@arm.com>;
> Ruifeng Wang <Ruifeng.Wang@arm.com>
> Cc: dev@dpdk.org
> Subject: Re: [PATCH v1 1/2] net/virtio: restrict pointer aliasing for NEON
> vpmd
>
>
>
> 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 virtio-net PVP case, and a 2.4% perf improvement in throughput
> > for the virtio-user PVP case. All performance data are measured
> > under the 0.001% acceptable packet loss with 2 cores on the vhost
> > side.
> >
> > Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> > Reviewed-by: Phil Yang <phil.yang@arm.com>
> > ---
> >  drivers/net/virtio/virtio_rxtx_simple_neon.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
>
> Very nice, we should consider doing the same on other platforms.
>
> > diff --git a/drivers/net/virtio/virtio_rxtx_simple_neon.c
> b/drivers/net/virtio/virtio_rxtx_simple_neon.c
> > index 363e2b330..c08dd51fb 100644
> > --- a/drivers/net/virtio/virtio_rxtx_simple_neon.c
> > +++ b/drivers/net/virtio/virtio_rxtx_simple_neon.c
> > @@ -36,8 +36,8 @@
> >   * - nb_pkts < RTE_VIRTIO_DESC_PER_LOOP, just return no packet
> >   */
> >  uint16_t
> > -virtio_recv_pkts_vec(void *rx_queue, struct rte_mbuf **rx_pkts,
> > -uint16_t nb_pkts)
> > +virtio_recv_pkts_vec(void *rx_queue, struct rte_mbuf
> > +**__restrict rx_pkts, uint16_t nb_pkts)
>
> Is __restrict supported by all the compilers?
> Wouldn't it be better to introduce a wrapper?

+1 for this.
In my understanding, the __restrict keyword is recognized in C at all language levels.
However, the restrict keyword is recognized in C under compilation with c99.
DPDK uses the restrict qualifier a lot, which might have some issues with some old compilers.
So the wrapper will be useful.

Thanks,
Phil

>
> >  {
> >  struct virtnet_rx *rxvq = rx_queue;
> >  struct virtqueue *vq = rxvq->vq;
> >

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