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Re: [PATCH] Too aggressive cwnd backoff

To: Stephen Hemminger <shemminger@xxxxxxxx>
Subject: Re: [PATCH] Too aggressive cwnd backoff
From: Werner Almesberger <werner@xxxxxxxxxxxxxxx>
Date: Thu, 7 Apr 2005 17:26:41 -0300
Cc: Baruch Even <baruch@xxxxxxxxx>, "David S. Miller" <davem@xxxxxxxxxxxxx>, netdev@xxxxxxxxxxx
In-reply-to: <20050407101653.2cc68db1@xxxxxxxxxxxxxxxxx>; from shemminger@xxxxxxxx on Thu, Apr 07, 2005 at 10:16:53AM -0700
References: <20050407164146.GA6479@xxxxxxxxx> <20050407101653.2cc68db1@xxxxxxxxxxxxxxxxx>
Sender: netdev-bounce@xxxxxxxxxxx
Stephen Hemminger wrote:
> I think this is a real problem, and was observed by Werner with umlsim.
> Don't know when it got introduced because it appears to pre-date the
> '04 work in adding Westwood, BIC, Vegas. Perhaps Alexey can shed some
> light on this.

If this is still the old cwnd quartering bug, then it's very very
old. Cheng Jin and I spotted it at the end of 2002 in 2.4.18, but
didn't look at earlier versions. (Later, I used this problem to
demonstrate how to fix bugs on the fly in umlsim.)

You can find the whole discussion in the following two threads:
http://thread.gmane.org/gmane.linux.network/2094
http://thread.gmane.org/gmane.linux.network/2223

Note that the fix isn't trivial (or, at least, it wasn't when we
looked at it), because just eliminating the /2 would make us too
aggressive in another, even weirder case, as I've described in
http://article.gmane.org/gmane.linux.network/2221
(which I admit to be somewhat daunting reading).

Recently, I noticed that my umlsim-based "hotfix" wouldn't improve
things in recent kernels anymore. I thought "cool, someone finally
sat down and fixed it", but perhaps that was premature.

- Werner

-- 
  _________________________________________________________________________
 / Werner Almesberger, Buenos Aires, Argentina     werner@xxxxxxxxxxxxxxx /
/_http://www.almesberger.net/____________________________________________/

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