Re: [PATCH 1/2] net/virtio: fix performance regression due to TSO enabling

[Olivier MATZ <olivier.matz@6wind.com>]

Hi Yuanhan,

Thanks for the fix.

On Wed, 11 Jan 2017 08:59:28 +0100, Maxime Coquelin
<maxime.coquelin@redhat.com> wrote:
> On 01/11/2017 05:27 AM, Yuanhan Liu wrote:
> > TSO is now enabled, but it's not actually being used by default in a
> > simple L2 forward mode. In such case, we have to zero the virtio net
> > headers, to inform the vhost backend that no offload is being used:
> >
> >     hdr->csum_start = 0;
> >     hdr->csum_offset = 0;
> >     hdr->flags = 0;
> >
> >     hdr->gso_type = 0;
> >     hdr->gso_size = 0;
> >     hdr->hdr_len = 0;
> >
> > Such writes could be very costly; it introduces severe cache issues:
> > The above operations introduce cache write for each packet, which
> > stalls the read operation from the vhost backend.
> >
> > The fact that virtio net header is initiated to zero in PMD driver
> > init stage means that these costly writes are unnecessary and could
> > be avoided:
> >
> >     if (hdr->csum_start != 0)
> >         hdr->csum_start = 0;
> >
> > And that's what the macro ASSIGN_UNLESS_EQUAL does. With this, the
> > performance drop introduced by TSO enabling is recovered: it could
> > be up to 20% in micro benchmarking.  
> Very nice!

Yep, I'm also impressed by the result.
I would have think this is something already done by the hardware and
transparent to the software.

> 
> >
> > Fixes: 58169a9c8153 ("net/virtio: support Tx checksum offload")
> > Fixes: 696573046e9e ("net/virtio: support TSO")
> >
> > Cc: Olivier Matz <olivier.matz@6wind.com>
> > Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: stable@dpdk.org
> > Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
>
> [...]
>
> Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Reviewed-by: Olivier Matz <olivier.matz@6wind.com>