> > Hi Ciara,
> >
> > I haven't tried this patch but have a question.
> >
> > On Thu, Aug 29, 2019 at 8:04 AM Ciara Loftus <ciara.lof...@intel.com>
> wrote:
> > >
> > > This patch enables the unaligned chunks feature for AF_XDP which
> > > allows chunks to be placed at arbitrary places in the umem, as
> > > opposed to them being required to be aligned to 2k. This allows for
> > > DPDK application mempools to be mapped directly into the umem and in
> > > turn enable zero copy transfer between umem and the PMD.
> > >
> > > This patch replaces the zero copy via external mbuf mechanism
> > > introduced in commit e9ff8bb71943 ("net/af_xdp: enable zero copy by
> > external mbuf").
> > > The pmd_zero copy vdev argument is also removed as now the PMD will
> > > auto-detect presence of the unaligned chunks feature and enable it
> > > if so and otherwise fall back to copy mode if not detected.
> > >
> > > When enabled, this feature significantly improves single-core
> > > performance of the PMD.
> >
> > Why using unaligned chunk feature improve performance?
> > Existing external mbuf already has zero copy between umem and PMD,
> and
> > your patch also does the same thing. So the improvement is from
> > somewhere else?
>
> Hi William,
>
> Good question.
> The external mbuf way indeed has zero copy however there's some
> additional complexity in that path in the management of the buf_ring.
>
> For example on the fill/rx path, in the ext mbuf solution one must dequeue
> an addr from the buf_ring and add it to the fill queue, allocate an mbuf for
> the external mbuf, get a pointer to the data @ addr and attach the external
> mbuf. With the new solution, we allocate an mbuf from the mempool, derive
> the addr from the mbuf itself and add it to the fill queue, and then on rx we
> can simply cast the pointer to the data @ addr to an mbuf and return it to the
> user.
> On tx/complete, instead of dequeuing from the buf_ring to get a valid addr
> we can again just derive it from the mbuf itself.
>
> I've performed some testing to compare the old vs new zc and found that for
> the case where the PMD and IRQs are pinned to separate cores the
> difference is ~-5%, but for single-core case where the PMD and IRQs are
> pinned to the same core (with the need_wakeup feature enabled), or when
> multiple PMDs are forwarding to one another the difference is significant.
> Please see below:
>
> ports queues/port pinning Δ old zc
> 1 1 0 -4.74%
> 1 1 1 17.99%
> 2 1 0 -5.62%
> 2 1 1 71.77%
> 1 2 0 114.24%
> 1 2 1 134.88%
Apologies, the last 4 figures above were comparing old memcpy vs zc. Corrected
data set below:
ports qs/port pinning Δ old zc
1 1 0 -4.74%
1 1 1 17.99%
2 1 0 -5.80%
2 1 1 37.24%
1 2 0 104.27%
1 2 1 136.73%
>
> FYI the series has been now merged into the bpf-next tree:
> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-
> next.git/commit/?id=bdb15a29cc28f8155e20f7fb58b60ffc452f2d1b
>
> Thanks,
> Ciara
>
> >
> > Thank you
> > William
> >
> > >
> > > Signed-off-by: Ciara Loftus <ciara.lof...@intel.com>
> > > Signed-off-by: Kevin Laatz <kevin.la...@intel.com>
> > > ---
> > > doc/guides/nics/af_xdp.rst | 1 -
> > > doc/guides/rel_notes/release_19_11.rst | 9 +
> > > drivers/net/af_xdp/rte_eth_af_xdp.c | 304 ++++++++++++++++++----
> --
> > -
> > > 3 files changed, 231 insertions(+), 83 deletions(-)
> > >
> > <snip>
