netdev - Re: [PATCH net-next 3/3] tcp: implement head drops in backlog queue

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Message-ID: <f86128c2-1109-a397-3a70-098556c21cea@gmail.com>
Date:   Wed, 21 Nov 2018 14:47:44 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Jean-Louis Dupond <jean-louis@...ond.be>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next 3/3] tcp: implement head drops in backlog queue



On 11/21/2018 02:40 PM, Yuchung Cheng wrote:
> On Wed, Nov 21, 2018 at 9:52 AM, Eric Dumazet <edumazet@...gle.com> wrote:
>> Under high stress, and if GRO or coalescing does not help,
>> we better make room in backlog queue to be able to keep latest
>> packet coming.
>>
>> This generally helps fast recovery, given that we often receive
>> packets in order.
> 
> I like the benefit of fast recovery but I am a bit leery about head
> drop causing HoLB on large read, while tail drops can be repaired by
> RACK and TLP already. Hmm -

This is very different pattern here.

We have a train of packets coming, the last packet is not a TLP probe...

Consider this train coming from an old stack without burst control nor pacing.

This patch guarantees last packet will be processed, and either :

1) We are a receiver, we will send a SACK. Sender will typically start recovery

2) We are a sender, we will process the most recent ACK sent by the receiver.