netdev - Re: Potential impact of commit dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win

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Date: Mon, 20 May 2024 19:20:49 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: "Subash Abhinov Kasiviswanathan (KS)" <quic_subashab@...cinc.com>
Cc: soheil@...gle.com, ncardwell@...gle.com, yyd@...gle.com, ycheng@...gle.com, 
	quic_stranche@...cinc.com, davem@...emloft.net, kuba@...nel.org, 
	netdev@...r.kernel.org
Subject: Re: Potential impact of commit dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")

On Mon, May 20, 2024 at 7:09 PM Subash Abhinov Kasiviswanathan (KS)
<quic_subashab@...cinc.com> wrote:
>
> On 5/20/2024 9:12 AM, Eric Dumazet wrote:
> > On Sun, May 19, 2024 at 4:14 AM Subash Abhinov Kasiviswanathan (KS)
> > <quic_subashab@...cinc.com> wrote:
> >>>>>>>>>>> We recently noticed that a device running a 6.6.17 kernel (A)
> >>>>>>>>>>> was having
> >>>>>>>>>>> a slower single stream download speed compared to a device running
> >>>>>>>>>>> 6.1.57 kernel (B). The test here is over mobile radio with
> >>>>>>>>>>> iperf3 with
> >>>>>>>>>>> window size 4M from a third party server.
> >>>>>>>>>>
> >>>>>>>>
> >>>>> This is not fixable easily, because tp->window_clamp has been
> >>>>> historically abused.
> >>>>>
> >>>>> TCP_WINDOW_CLAMP socket option should have used a separate tcp socket
> >>>>> field
> >>>>> to remember tp->window_clamp has been set (fixed) to a user value.
> >>>>>
> >>>>> Make sure you have this followup patch, dealing with applications
> >>>>> still needing to make TCP slow.
> >>>>>
> >>>>> commit 697a6c8cec03c2299f850fa50322641a8bf6b915
> >>>>> Author: Hechao Li <hli@...flix.com>
> >>>>> Date:   Tue Apr 9 09:43:55 2024 -0700
> >>>>>
> >>>>>       tcp: increase the default TCP scaling ratio
> >>> With 4M SO_RCVBUF, the receiver window scaled to ~4M. Download speed
> >>> increased significantly but didn't match the download speed of B with 4M
> >>> SO_RCVBUF. Per commit description, the commit matches the behavior as if
> >>> tcp_adv_win_scale was set to 1.
> >>>
> >>> Download speed of B is higher than A for 4M SO_RCVBUF as receiver window
> >>> of B grew to ~6M. This is because B had tcp_adv_win_scale set to 2.
> >> Would the following to change to re-enable the use of sysctl
> >> tcp_adv_win_scale to set the initial scaling ratio be acceptable.
> >> Default value of tcp_adv_win_scale is 1 which corresponds to the
> >> existing 50% ratio.
> >>
> >> I verified with this patch on A that setting SO_RCVBUF 4M in iperf3 with
> >> tcp_adv_win_scale = 1 (default) scales receiver window to ~4M while
> >> tcp_adv_win_scale = 2 scales receiver window to ~6M (which matches the
> >> behavior from B).
> >
> > What problem are you trying to solve that commit  697a6c8cec03c229
> > did not ?
> >
> Commit 697a6c8cec03c229 added support to increase initial scaling ratio
> to 50% to match behavior of configurations which were using SO_RCVBUF
> and only tcp_adv_win_scale = 1 (prior to commit dfa2f0483360).
>
> My proposed change is adding support for configurations which are using
> SO_RCVBUF and other tcp_adv_win_scale values. In my case, device B was
> using SO_RCVBUF and tcp_adv_win_scale = 2.

I do not think we want to bring back a config option that has been
superseded by something
allowing a host to have multiple NIC, with different MTU, and multiple
TCP flows with various MSS.