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Date:   Wed, 1 Jul 2020 15:07:06 +0200
From:   Matthieu Baerts <matthieu.baerts@...sares.net>
To:     Florian Westphal <fw@...len.de>, netdev@...r.kernel.org
Cc:     mptcp@...ts.01.org
Subject: Re: [PATCH net-next 2/2] mptcp: add receive buffer auto-tuning

Hi Florian,

On 30/06/2020 21:24, Florian Westphal wrote:
> When mptcp is used, userspace doesn't read from the tcp (subflow)
> socket but from the parent (mptcp) socket receive queue.
> 
> skbs are moved from the subflow socket to the mptcp rx queue either from
> 'data_ready' callback (if mptcp socket can be locked), a work queue, or
> the socket receive function.
> 
> This means tcp_rcv_space_adjust() is never called and thus no receive
> buffer size auto-tuning is done.
> 
> An earlier (not merged) patch added tcp_rcv_space_adjust() calls to the
> function that moves skbs from subflow to mptcp socket.
> While this enabled autotuning, it also meant tuning was done even if
> userspace was reading the mptcp socket very slowly.
> 
> This adds mptcp_rcv_space_adjust() and calls it after userspace has
> read data from the mptcp socket rx queue.
> 
> Its very similar to tcp_rcv_space_adjust, with two differences:
> 
> 1. The rtt estimate is the largest one observed on a subflow
> 2. The rcvbuf size and window clamp of all subflows is adjusted
>     to the mptcp-level rcvbuf.
> 
> Otherwise, we get spurious drops at tcp (subflow) socket level if
> the skbs are not moved to the mptcp socket fast enough.
> 
> Before:
> time mptcp_connect.sh -t -f $((4*1024*1024)) -d 300 -l 0.01% -r 0 -e "" -m mmap
> [..]
> ns4 MPTCP -> ns3 (10.0.3.2:10108      ) MPTCP   (duration 40823ms) [ OK ]
> ns4 MPTCP -> ns3 (10.0.3.2:10109      ) TCP     (duration 23119ms) [ OK ]
> ns4 TCP   -> ns3 (10.0.3.2:10110      ) MPTCP   (duration  5421ms) [ OK ]
> ns4 MPTCP -> ns3 (dead:beef:3::2:10111) MPTCP   (duration 41446ms) [ OK ]
> ns4 MPTCP -> ns3 (dead:beef:3::2:10112) TCP     (duration 23427ms) [ OK ]
> ns4 TCP   -> ns3 (dead:beef:3::2:10113) MPTCP   (duration  5426ms) [ OK ]
> Time: 1396 seconds
> 
> After:
> ns4 MPTCP -> ns3 (10.0.3.2:10108      ) MPTCP   (duration  5417ms) [ OK ]
> ns4 MPTCP -> ns3 (10.0.3.2:10109      ) TCP     (duration  5427ms) [ OK ]
> ns4 TCP   -> ns3 (10.0.3.2:10110      ) MPTCP   (duration  5422ms) [ OK ]
> ns4 MPTCP -> ns3 (dead:beef:3::2:10111) MPTCP   (duration  5415ms) [ OK ]
> ns4 MPTCP -> ns3 (dead:beef:3::2:10112) TCP     (duration  5422ms) [ OK ]
> ns4 TCP   -> ns3 (dead:beef:3::2:10113) MPTCP   (duration  5423ms) [ OK ]
> Time: 296 seconds
> 
> Signed-off-by: Florian Westphal <fw@...len.de>

Thank you for adding this very useful feature!

Reviewed-by: Matthieu Baerts <matthieu.baerts@...sares.net>

Cheers,
Matt
-- 
Tessares | Belgium | Hybrid Access Solutions
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