Re: [dpdk-dev] [PATCH] net/af_xdp: enable support for unaligned umem chunks

[William Tu <u9012063@gmail.com>]

On Mon, Sep 2, 2019 at 1:55 AM Loftus, Ciara <ciara.loftus@intel.com> wrote:
>
> > > Hi Ciara,
> > >
> > > I haven't tried this patch but have a question.
> > >
> > > On Thu, Aug 29, 2019 at 8:04 AM Ciara Loftus <ciara.loftus@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
> >

Great, thanks for sharing the number and explanation.
William

> > Thanks,
> > Ciara
> >
> > >
> > > Thank you
> > > William
> > >
> > > >
> > > > Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
> > > > Signed-off-by: Kevin Laatz <kevin.laatz@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>