netdev - Re: [PATCH net v2 1/3] tcp_cubic: fix to run bictcp

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Message-ID: <CADVnQymaehUnVtqhraet3tryvMyJCbA1f7FCb+h4EbbdR+oOzw@mail.gmail.com>
Date: Thu, 15 Aug 2024 10:46:36 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Mingrui Zhang <mrzhang97@...il.com>
Cc: edumazet@...gle.com, davem@...emloft.net, netdev@...r.kernel.org, 
	Lisong Xu <xu@....edu>
Subject: Re: [PATCH net v2 1/3] tcp_cubic: fix to run bictcp_update() at least
 once per RTT

On Wed, Aug 14, 2024 at 8:19 PM Mingrui Zhang <mrzhang97@...il.com> wrote:
>
> The original code bypasses bictcp_update() under certain conditions
> to reduce the CPU overhead. Intuitively, when last_cwnd==cwnd,
> bictcp_update() is executed 32 times per second. As a result,
> it is possible that bictcp_update() is not executed for several
> RTTs when RTT is short (specifically < 1/32 second = 31 ms and
> last_cwnd==cwnd which may happen in small-BDP networks),
> thus leading to low throughput in these RTTs.
>
> The patched code executes bictcp_update() 32 times per second
> if RTT > 31 ms or every RTT if RTT < 31 ms, when last_cwnd==cwnd.
>
> Thanks
> Mingrui, and Lisong

Another note: please remove these commit message lines like this with
"Thanks" and your names, from each of the 3 commits.

Please run "git log" in your git Linux working directory to get a
sense of the conventions for Linux commit messages. :-)

Thanks!
neal


> Fixes: 91a4599c2ad8 ("tcp_cubic: fix to run bictcp_update() at least once per RTT")
> Signed-off-by: Mingrui Zhang <mrzhang97@...il.com>
> Signed-off-by: Lisong Xu <xu@....edu>
>
> ---
>  net/ipv4/tcp_cubic.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
> index 5dbed91c6178..11bad5317a8f 100644
> --- a/net/ipv4/tcp_cubic.c
> +++ b/net/ipv4/tcp_cubic.c
> @@ -218,8 +218,12 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
>
>         ca->ack_cnt += acked;   /* count the number of ACKed packets */
>
> +       /* Update 32 times per second if RTT > 1/32 second,
> +        *        every RTT if RTT < 1/32 second
> +        *        even when last_cwnd == cwnd
> +        */
>         if (ca->last_cwnd == cwnd &&
> -           (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
> +           (s32)(tcp_jiffies32 - ca->last_time) <= min(HZ / 32, usecs_to_jiffies(ca->delay_min)))
>                 return;
>
>         /* The CUBIC function can update ca->cnt at most once per jiffy.
> --
> 2.34.1
>