netdev - Re: [PATCH v3 net-next 3/6] tcp: add SACK compression

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Message-ID: <CADVnQykUpdC+2e94_x9v9iX6RC7ro=bkvH5Q7-p0pbtBhHyU=A@mail.gmail.com>
Date:   Thu, 17 May 2018 17:57:16 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v3 net-next 3/6] tcp: add SACK compression

On Thu, May 17, 2018 at 5:47 PM Eric Dumazet <edumazet@...gle.com> wrote:

> When TCP receives an out-of-order packet, it immediately sends
> a SACK packet, generating network load but also forcing the
> receiver to send 1-MSS pathological packets, increasing its
> RTX queue length/depth, and thus processing time.

> Wifi networks suffer from this aggressive behavior, but generally
> speaking, all these SACK packets add fuel to the fire when networks
> are under congestion.

> This patch adds a high resolution timer and tp->compressed_ack counter.

> Instead of sending a SACK, we program this timer with a small delay,
> based on RTT and capped to 1 ms :

>          delay = min ( 5 % of RTT, 1 ms)

> If subsequent SACKs need to be sent while the timer has not yet
> expired, we simply increment tp->compressed_ack.

> When timer expires, a SACK is sent with the latest information.
> Whenever an ACK is sent (if data is sent, or if in-order
> data is received) timer is canceled.

> Note that tcp_sack_new_ofo_skb() is able to force a SACK to be sent
> if the sack blocks need to be shuffled, even if the timer has not
> expired.

> A new SNMP counter is added in the following patch.

> Two other patches add sysctls to allow changing the 1,000,000 and 44
> values that this commit hard-coded.

> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---

Very nice. I like the constants and the min(rcv_rtt, srtt).

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

Thanks!

neal