I've been seeing the same problem here and am trying to fix it.
My fix is to not count those pkts still in the host queue as "prior_in_flight"
when feeding the latter to tcp_cong_avoid(). This should cause
tcp_is_cwnd_limited() test to fail when the previous in_flight build-up
is all due to the large host queue, and stop the cwnd to grow beyond
what's really necessary.

"sysctl_tcp_tso_win_divisor causes cwnd" also unnecessarily inflates
cwnd quite a bit when TSO is enabled.

Jerry

From: David Miller <davem@davemloft.net>

quoted text
>
>
> Date: Tue, Apr 22, 2008 at 8:59 PM
>
> Subject: Re: Socket buffer sizes with autotuning
> To: johnwheffner@gmail.com
> Cc: rick.jones2@hp.com, netdev@vger.kernel.org
>
>
> From: "John Heffner" <johnwheffner@gmail.com>
> Date: Tue, 22 Apr 2008 19:17:39 -0700
>
>
>
> > On Tue, Apr 22, 2008 at 5:38 PM, Rick Jones <rick.jones2@hp.com> wrote:
> > >  oslowest:~# netstat -an | grep ESTAB
> > >  ...
> > >  tcp    0 2760560 10.208.0.1:40500    10.208.0.45:42049   ESTABLISHED
> > >  ...
> > >
> > >  Is this expected behaviour?
> >
>
> > What is your interface txqueuelen and mtu?  If you have a very large
> > interface queue, TCP will happily fill it up unless you are using a
> > delay-based congestion controller.
>
>
> Yes, that's the fundamental problem with loss based congestion
> control.  If there are any queues in the path, TCP will fill them up.
>
> Vegas and other similar techniques are able to avoid this, but come
> with the fundamental flaw that it's easy to get them into situations
> where they do not respond to increases in pipe space adequately, and
> thus underperform compared to loss based algorithms.
>
>
>
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