netdev - Re: [PATCH net-next 3/3] tcp: derive delack_max from rto

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Date: Wed, 20 Sep 2023 15:06:53 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Soheil Hassas Yeganeh <soheil@...gle.com>, Yuchung Cheng <ycheng@...gle.com>, 
	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next 3/3] tcp: derive delack_max from rto_min

On Wed, Sep 20, 2023 at 1:29 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> While BPF allows to set icsk->->icsk_delack_max
> and/or icsk->icsk_rto_min, we have an ip route
> attribute (RTAX_RTO_MIN) to be able to tune rto_min,
> but nothing to consequently adjust max delayed ack,
> which vary from 40ms to 200 ms (TCP_DELACK_{MIN|MAX}).
>
> This makes RTAX_RTO_MIN of almost no practical use,
> unless customers are in big trouble.
>
> Modern days datacenter communications want to set
> rto_min to ~5 ms, and the max delayed ack one jiffie
> smaller to avoid spurious retransmits.
>
> After this patch, an "rto_min 5" route attribute will
> effectively lower max delayed ack timers to 4 ms.
>
> Note in the following ss output, "rto:6 ... ato:4"
>
> $ ss -temoi dst XXXXXX
> State Recv-Q Send-Q           Local Address:Port       Peer Address:Port  Process
> ESTAB 0      0        [2002:a05:6608:295::]:52950   [2002:a05:6608:297::]:41597
>      ino:255134 sk:1001 <->
>          skmem:(r0,rb1707063,t872,tb262144,f0,w0,o0,bl0,d0) ts sack
>  cubic wscale:8,8 rto:6 rtt:0.02/0.002 ato:4 mss:4096 pmtu:4500
>  rcvmss:536 advmss:4096 cwnd:10 bytes_sent:54823160 bytes_acked:54823121
>  bytes_received:54823120 segs_out:1370582 segs_in:1370580
>  data_segs_out:1370579 data_segs_in:1370578 send 16.4Gbps
>  pacing_rate 32.6Gbps delivery_rate 1.72Gbps delivered:1370579
>  busy:26920ms unacked:1 rcv_rtt:34.615 rcv_space:65920
>  rcv_ssthresh:65535 minrtt:0.015 snd_wnd:65536
>
> While we could argue this patch fixes a bug with RTAX_RTO_MIN,
> I do not add a Fixes: tag, so that we can soak it a bit before
> asking backports to stable branches.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Eric!

neal