netdev - Re: [PATCH net v3] tcp_cubic: fix incorrect HyStart round start detection

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Message-ID: <CANn89i+g380KQq7C8GEJVxwNNZJE6gwq3JCCyGsn6M09+y8N7Q@mail.gmail.com>
Date: Fri, 17 Jan 2025 23:00:17 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Mahdi Arghavani <ma.arghavani@...oo.com>
Cc: netdev@...r.kernel.org, ncardwell@...gle.com, haibo.zhang@...go.ac.nz, 
	david.eyers@...go.ac.nz, abbas.arghavani@....se, 
	Jason Xing <kerneljasonxing@...il.com>
Subject: Re: [PATCH net v3] tcp_cubic: fix incorrect HyStart round start detection

On Fri, Jan 17, 2025 at 10:38 PM Mahdi Arghavani <ma.arghavani@...oo.com> wrote:
>
> I noticed that HyStart incorrectly marks the start of rounds,
> leading to inaccurate measurements of ACK train lengths and
> resetting the `ca->sample_cnt` variable. This inaccuracy can impact
> HyStart's functionality in terminating exponential cwnd growth during
> Slow-Start, potentially degrading TCP performance.
>
> The issue arises because the changes introduced in commit 4e1fddc98d25
> ("tcp_cubic: fix spurious Hystart ACK train detections for not-cwnd-limited flows")
> moved the caller of the `bictcp_hystart_reset` function inside the `hystart_update` function.
> This modification added an additional condition for triggering the caller,
> requiring that (tcp_snd_cwnd(tp) >= hystart_low_window) must also
> be satisfied before invoking `bictcp_hystart_reset`.
>
> This fix ensures that `bictcp_hystart_reset` is correctly called
> at the start of a new round, regardless of the congestion window size.
> This is achieved by moving the condition
> (tcp_snd_cwnd(tp) >= hystart_low_window)
> from before calling `bictcp_hystart_reset` to after it.
>
> I tested with a client and a server connected through two Linux software routers.
> In this setup, the minimum RTT was 150 ms, the bottleneck bandwidth was 50 Mbps,
> and the bottleneck buffer size was 1 BDP, calculated as (50M / 1514 / 8) * 0.150 = 619 packets.
> I conducted the test twice, transferring data from the server to the client for 1.5 seconds.
> Before the patch was applied, HYSTART-DELAY stopped the exponential growth of cwnd when
> cwnd = 516, and the bottleneck link was not yet saturated (516 < 619).
> After the patch was applied, HYSTART-ACK-TRAIN stopped the exponential growth of cwnd when
> cwnd = 632, and the bottleneck link was saturated (632 > 619).
> In this test, applying the patch resulted in 300 KB more data delivered.
>
> Fixes: 4e1fddc98d25 ("tcp_cubic: fix spurious Hystart ACK train detections for not-cwnd-limited flows")
> Signed-off-by: Mahdi Arghavani <ma.arghavani@...oo.com>
> Reviewed-by: Jason Xing <kerneljasonxing@...il.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Haibo Zhang <haibo.zhang@...go.ac.nz>
> Cc: David Eyers <david.eyers@...go.ac.nz>
> Cc: Abbas Arghavani <abbas.arghavani@....se>
> ---

SGTM thanks.

Reviewed-by: Eric Dumazet <edumazet@...gle.com>