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From: Neal Cardwell <ncardwell@google.com>
To: Dave Taht <dave.taht@gmail.com>
Cc: bloat <bloat@lists.bufferbloat.net>, Rpm <rpm@lists.bufferbloat.net>
Subject: Re: [Bloat] *under*bloated networks
Date: Sun, 28 Nov 2021 10:42:25 -0500	[thread overview]
Message-ID: <CADVnQynenp-VJ=ECm5QBvSQ1Y-uyj95=XftDNqXuWdHCDd02yQ@mail.gmail.com> (raw)
In-Reply-To: <CAA93jw6fz2REDiFAdfrdcAmgQ0rcFDDS0TQqMxgLeGxHK1nBKw@mail.gmail.com>


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On Mon, Nov 22, 2021 at 10:35 PM Dave Taht <dave.taht@gmail.com> wrote:

> In the last two weeks I have found two dramatically underbuffered Gbit
> fiber networks.
>
> This one appears to have about a 400 full size packet uplink buffer
> (5ms)[1]
>
> https://imgur.com/a/Bm9hdNf
>
> It was pretty remarkable to see how well multiple tcp flows still
> achieved close to the full rate with such a small fixed size queue,
> eventually.
>
> A single bbr flow can't crack 150mbits: https://imgur.com/a/DpydL5K.
>

Thanks, Dave. The single-flow BBR upload case is interesting.

I took a look at the packet traces in the later thread:
  https://www.reddit.com/r/eero/comments/qxbkcl/66_is_out/hltlep0/

For the single-flow BBR case it seems that....

(1) the BBR(v1) flow is running into a 300 Mbps bottleneck rate (visible in
the slope of the green ACK line in the zoomed-in trace, attached).

(2) the BBR(v1) flow is achieving an average rate a bit above 150 Mbps
because it repeatedly runs into receive window limits (the yellow line in
the zoomed-out trace, attached). The frequent receive window limits mean
that the flow spends a lot of time unable to send anything, thus leading to
lower average throughput.

cheers,
neal

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  reply	other threads:[~2021-11-28 15:42 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-23  3:35 Dave Taht
2021-11-28 15:42 ` Neal Cardwell [this message]
2021-11-28 18:24   ` Dave Taht

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