netdev - Re: [PATCH net-next] net-tcp: Disable TCP ssthresh metrics cache by default

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Date:   Mon, 09 Dec 2019 20:18:03 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     yyd@...gle.com
Cc:     netdev@...r.kernel.org, edumazet@...gle.com, ncardwell@...gle.com,
        ycheng@...gle.com
Subject: Re: [PATCH net-next] net-tcp: Disable TCP ssthresh metrics cache
 by default

From: "Kevin(Yudong) Yang" <yyd@...gle.com>
Date: Mon,  9 Dec 2019 14:19:59 -0500

> This patch introduces a sysctl knob "net.ipv4.tcp_no_ssthresh_metrics_save"
> that disables TCP ssthresh metrics cache by default. Other parts of TCP
> metrics cache, e.g. rtt, cwnd, remain unchanged.
> 
> As modern networks becoming more and more dynamic, TCP metrics cache
> today often causes more harm than benefits. For example, the same IP
> address is often shared by different subscribers behind NAT in residential
> networks. Even if the IP address is not shared by different users,
> caching the slow-start threshold of a previous short flow using loss-based
> congestion control (e.g. cubic) often causes the future longer flows of
> the same network path to exit slow-start prematurely with abysmal
> throughput.
> 
> Caching ssthresh is very risky and can lead to terrible performance.
> Therefore it makes sense to make disabling ssthresh caching by
> default and opt-in for specific networks by the administrators.
> This practice also has worked well for several years of deployment with
> CUBIC congestion control at Google.
> 
> Acked-by: Eric Dumazet <edumazet@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Acked-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Kevin(Yudong) Yang <yyd@...gle.com>

Makes sense, applied, thanks.