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

[William Tu <u9012063@gmail.com>]

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?

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>