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

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Date: Mon, 20 May 2024 11:33:14 -0600
From: "Subash Abhinov Kasiviswanathan (KS)" <quic_subashab@...cinc.com>
To: Eric Dumazet <edumazet@...gle.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 5/20/2024 11:20 AM, Eric Dumazet wrote:
> 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).
> 
> 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.
The default value still stays 1 and all of the accurate estimation of 
skb->len/skb->truesize still remains for all auto tuning users. I 
believe that should continue support all the configurations you mentioned.

I merely want to add the flexibility for users which have been affected 
here due to lack of backwards compatibility (SO_RCVBUF with 
tcp_adv_win_scale value other than 1).