netdev - Re: [PATCH v2] tcp: adjust rcvbuff according copied rate of user space

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Date:   Wed, 10 Aug 2022 08:43:44 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Yonglong Li <liyonglong@...natelecom.cn>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        ycheng@...gle.com, dsahern@...nel.org, kuba@...nel.org,
        pabeni@...hat.com
Subject: Re: [PATCH v2] tcp: adjust rcvbuff according copied rate of user space

On Wed, Aug 10, 2022 at 3:49 AM Yonglong Li <liyonglong@...natelecom.cn> wrote:
>
> every time data is copied to user space tcp_rcv_space_adjust is called.
> current It adjust rcvbuff by the length of data copied to user space.
> If the interval of user space copy data from socket is not stable, the
> length of data copied to user space will not exactly show the speed of
> copying data from rcvbuff.
> so in tcp_rcv_space_adjust it is more reasonable to adjust rcvbuff by
> copied rate (length of copied data/interval)instead of copied data len
>
> I tested this patch in simulation environment by Mininet:
> with 80~120ms RTT / 1% loss link, 100 runs
> of (netperf -t TCP_STREAM -l 5), and got an average throughput
> of 17715 Kbit instead of 17703 Kbit.
> with 80~120ms RTT without loss link, 100 runs of (netperf -t
> TCP_STREAM -l 5), and got an average throughput of 18272 Kbit
> instead of 18248 Kbit.

So with 1% emulated loss that's a 0.06% throughput improvement and
without emulated loss that's a 0.13% improvement. That sounds like it
may well be statistical noise, particularly given that we would expect
the steady-state impact of this change to be negligible.

IMHO these results do not justify the added complexity and state.

best regards,
neal