Re: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for packed vring desc avail flags

["Wang, Yinan" <yinan.wang@intel.com>]

Hi Joyce,

I just test performance impact of your patch set with code base commit id: d03d8622db48918d14bfe805641b1766ecc40088, after applying your v3 patch set , seven paths of vhost/virtio pvp test shows performance drop as below:

PVP vhost/virtio 1c1q test	         before apply patch	apply patch
test_perf_pvp_inorder_mergeable     	 7.603	           7.474
test_perf_pvp_inorder_no_mergeable	     7.642	           7.525
test_perf_pvp_mergeable	              7.556	           7.431
test_perf_pvp_normal	                   7.554	           7.478
test_perf_pvp_vector_rx	               7.581	           7.469
test_perf_pvp_virtio11_mergeable	           7.068	           6.905
test_perf_pvp_virtio11_normal	           7.088	           6.888

Thanks,
Yinan

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Maxime Coquelin
> Sent: 2019年9月9日 18:10
> To: Joyce Kong <joyce.kong@arm.com>; dev@dpdk.org
> Cc: nd@arm.com; Bie, Tiwei <tiwei.bie@intel.com>; Wang, Zhihong
> <zhihong.wang@intel.com>; amorenoz@redhat.com; Wang, Xiao W
> <xiao.w.wang@intel.com>; Liu, Yong <yong.liu@intel.com>;
> jfreimann@redhat.com; honnappa.nagarahalli@arm.com; gavin.hu@arm.com
> Subject: Re: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for packed vring
> desc avail flags
> 
> 
> 
> On 9/9/19 11:14 AM, Joyce Kong wrote:
> > In case VIRTIO_F_ORDER_PLATFORM(36) is not negotiated, then the
> > frontend and backend are assumed to be implemented in software, that
> > is they can run on identical CPUs in an SMP configuration.
> > Thus a weak form of memory barriers like rte_smp_r/wmb, other than
> > rte_cio_r/wmb, is sufficient for this case(vq->hw->weak_barriers == 1)
> > and yields better performance.
> > For the above case, this patch helps yielding even better performance
> > by replacing the two-way barriers with C11 one-way barriers for avail
> > flags in packed ring.
> >
> > Meanwhile, a read barrier is required to ensure ordering between
> > descriptor's flags and content reads[1]. With C11, load-acquire can
> > enforce the ordering instead of rmb barrier.
> >
> > [1]https://patchwork.dpdk.org/patch/49109/
> >
> > Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> > Reviewed-by: Phil Yang <phil.yang@arm.com>
> > ---
> >  drivers/net/virtio/virtio_rxtx.c                 | 13 +++++++------
> >  drivers/net/virtio/virtio_user/virtio_user_dev.c |  6 +++++-
> >  drivers/net/virtio/virtqueue.h                   | 11 +++++++++++
> >  lib/librte_vhost/vhost.h                         |  2 +-
> >  lib/librte_vhost/virtio_net.c                    | 11 +++++------
> >  5 files changed, 29 insertions(+), 14 deletions(-)
> 
> Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> 
> Thanks,
> Maxime