Re: [virtio-dev] packed ring layout proposal

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Michael,

On 09/19/2016 07:24 PM, Michael S. Tsirkin wrote:
> On Mon, Sep 19, 2016 at 10:03:36AM -0700, Stephen Hemminger wrote:
> > On Sun, 18 Sep 2016 08:36:05 -0400 (EDT)
> > Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > > > Without indirect the usable ring size is cut in half on older kernels that
> > > > don't support any layout. That limit on qemu ends up being 128 packets
> > >
> > > Hi Stephen,
> > >
> > > note that here we were talking about limiting direct descriptors to
> > > requests with a single buffer.
> > >
> > > Paolo
> >
> > The test I was looking at was small packet transmit (like pktgen or RFC2544).
> > The idea was to make the DPDK driver use all the same algorithms as the Linux
> > kernel which is the baseline for virtio. Also handling jumbo MTU packets
> > efficiently with scatter/gather.
> >
> > With DPDK the issue was that the typical packet required several transmit
> > slot entries. Particularly bad for jumbo packets which usually are made up
> > of several mbuf's. A 9K packet typically has 5 mbuf's plus one more for the
> > virtio header.
> >
> > Ideally, the DPDK driver should use the best algorithm it can negotiate.
> > Be sure and test with old RHEL6 type systems, modern enterprise (RHEL/SLES)
> > and also Google Compute which has there own virtio.  Not sure what virtual
> > box is doing??  Why can't code be written like ixgbe driver which has a
> > fast path if config allows it but makes decision at run time.
>
> Interesting.  Maxime here posted integrating the indirect buffer
> support to address this but this didn't get accepted
> since he could not prove the performance gain.
>
> Maxime, could you share the link to your patch so we
> can discuss this on the dpdk mailing list?
Here it is:
http://dpdk.org/dev/patchwork/patch/14797/

Thanks,
Maxime

> Thanks!