netdev - Re: [Make-wifi-fast] debugging TCP stalls on high-speed wifi

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Message-Id: <5DBB113A-E61A-4032-B877-18BE3ABBC616@superduper.net>
Date:   Thu, 12 Dec 2019 17:57:13 -0800
From:   Simon Barber <simon@...erduper.net>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Dave Taht <dave.taht@...il.com>,
        Make-Wifi-fast <make-wifi-fast@...ts.bufferbloat.net>,
        Johannes Berg <johannes@...solutions.net>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [Make-wifi-fast] debugging TCP stalls on high-speed wifi

In my application this is a bridge or router (not TCP endpoint), and the driver is doing GRO and NAPI polling. Also looking at using the skb->fraglist to make the GRO code more effective and more transparent by passing flags, short segments, etc through for perfect reconstruction by TSO.

Simon

> On Dec 12, 2019, at 5:46 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> 
> 
> On 12/12/19 4:59 PM, Simon Barber wrote:
>> I’m currently adding ACK thinning to Linux’s GRO code. Quite a simple addition given the way that code works.
>> 
>> Simon
>> 
>> 
> 
> Please don't.
> 
> 1) It will not help since many NIC  do not use GRO.
> 
> 2) This does not help if you receive one ACK per NIC interrupt, which is quite common.
> 
> 3) This breaks GRO transparency.
> 
> 4) TCP can implement this in a more effective/controlled way,
>   since the peer know a lot more flow characteristics.
> 
> Middle-box should not try to make TCP better, they usually break things.