Re: [RFC PATCH net-next] net: gro: coalesce padded small IPv4 TCP segments

[Glenn Judd]

On Mon, Aug 10, 2026 at 6:11 AM Richard Gobert <richardbgobert@xxxxxxxxx> wrote:
>
> To address Jakub's question on the fast-path cost: I benchmarked GRO
> forwarding with two-minute long iperf sessions using 1, 2 and 4 TCP streams
> (17 runs per configuration) and CPU frequency scaling disabled. I also
> disabled RSS during the benchmarks because it caused a lot of noise - up to
> 20% variance in the deltas.
>
> | streams | baseline (Gbit/s)  | patched (Gbit/s)  | delta  |
> |---------|--------------------|-------------------|--------|
> |    1    | 14.083 ± 1.01      | 14.065 ± 0.67     | −0.13% |
> |    2    | 13.891 ± 0.75      | 13.926 ± 0.78     | +0.25% |
> |    4    | 13.008 ± 1.26      | 13.029 ± 0.97     | +0.16% |
>
> The two added fast-path operations (iph->tot_len read + the skb_gro_len()
> comparison on every IPv4 GRO packet) produce no measurable throughput
> change. The deltas are all well under the 95% confidence interval and
> indistinguishable from noise.

Thanks for that analysis. It's in the ballpark of what I see on my
side, though I do see a small cost in my setup. I'll follow up with a
revised approach in v2.