Re: [dpdk-dev] [PATCH v2 1/3] examples/l3fwd: reorganize code for better performance

[Jerin Jacob <jerinjacobk@gmail.com>]

On Tue, Jun 1, 2021 at 1:27 PM Ruifeng Wang <ruifeng.wang@arm.com> wrote:
>
> Moved rfc1812 process prior to NEON registers store.
> On N1SDP, this reorganization mitigates CPU frontend stall and backend
> stall when forwarding.
>
> On N1SDP with MLX5 40G NIC, this change showed 10.2% performance gain
> in single port single core MRR test.

I think, it may not have anything to do with N1SDP, It could be just
the prefetch window timing
with MLX5 driver on Tx mbuf on touching  with tx_burst() and L1 cache
pressure timing.
I think, tuning the driver parameters can switch the window to some driver code.

On Octeontx2, this change has regression of -3.1% flow lookup miss
case. so NACK.


> On ThunderX2, this changed showed no performance degradation.
>
> Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
>  examples/l3fwd/l3fwd_neon.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/examples/l3fwd/l3fwd_neon.h b/examples/l3fwd/l3fwd_neon.h
> index 86ac5971d7..ea7fe22d00 100644
> --- a/examples/l3fwd/l3fwd_neon.h
> +++ b/examples/l3fwd/l3fwd_neon.h
> @@ -43,11 +43,6 @@ processx4_step3(struct rte_mbuf *pkt[FWDSTEP], uint16_t dst_port[FWDSTEP])
>         ve[2] = vsetq_lane_u32(vgetq_lane_u32(te[2], 3), ve[2], 3);
>         ve[3] = vsetq_lane_u32(vgetq_lane_u32(te[3], 3), ve[3], 3);
>
> -       vst1q_u32(p[0], ve[0]);
> -       vst1q_u32(p[1], ve[1]);
> -       vst1q_u32(p[2], ve[2]);
> -       vst1q_u32(p[3], ve[3]);
> -
>         rfc1812_process((struct rte_ipv4_hdr *)
>                         ((struct rte_ether_hdr *)p[0] + 1),
>                         &dst_port[0], pkt[0]->packet_type);
> @@ -60,6 +55,11 @@ processx4_step3(struct rte_mbuf *pkt[FWDSTEP], uint16_t dst_port[FWDSTEP])
>         rfc1812_process((struct rte_ipv4_hdr *)
>                         ((struct rte_ether_hdr *)p[3] + 1),
>                         &dst_port[3], pkt[3]->packet_type);
> +
> +       vst1q_u32(p[0], ve[0]);
> +       vst1q_u32(p[1], ve[1]);
> +       vst1q_u32(p[2], ve[2]);
> +       vst1q_u32(p[3], ve[3]);
>  }
>
>  /*
> --
> 2.25.1
>