netdev - Re: Socket buffer sizes with autotuning

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Message-ID: <d1c2719f0804231629p4536e98do668b812e34d2b92c@mail.gmail.com>
Date:	Wed, 23 Apr 2008 16:29:58 -0700
From:	"Jerry Chu" <hkchu@...gle.com>
To:	netdev@...r.kernel.org
Subject: Re: Socket buffer sizes with autotuning

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@...emloft.net>

>
>
> Date: Tue, Apr 22, 2008 at 8:59 PM
>
> Subject: Re: Socket buffer sizes with autotuning
> To: johnwheffner@...il.com
> Cc: rick.jones2@...com, netdev@...r.kernel.org
>
>
> From: "John Heffner" <johnwheffner@...il.com>
> Date: Tue, 22 Apr 2008 19:17:39 -0700
>
>
>
> > On Tue, Apr 22, 2008 at 5:38 PM, Rick Jones <rick.jones2@...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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