netdev - Re: TCP rx window autotuning harmful at LAN context

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Message-Id: <20090309.170927.130334650.davem@davemloft.net>
Date:	Mon, 09 Mar 2009 17:09:27 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	md@....sk
Cc:	netdev@...r.kernel.org
Subject: Re: TCP rx window autotuning harmful at LAN context

From: Marian Ďurkovič <md@....sk>
Date: Mon, 9 Mar 2009 21:05:05 +0100

> Well, in practice that was always limited by receive window size, which
> was by default 64 kB on most operating systems. So this undesirable behavior
> was limited to hosts where receive window was manually increased to huge
> values.

You say "was" as if this was a recent change.  Linux has been doing
receive buffer autotuning for at least 5 years if not longer.

> Today, the real effect of autotuning is the same as changing the
> receive window size to 4 MB on *all* hosts, since there's no
> mechanism to prevent it from growing the window to maximum even for
> low RTT paths.

There is, on the sender side (congestion control) and at the
intermediate bottleneck routers (active queue management).

You are pointing the blame at the wrong area, as both John and Stephen
are trying to tell you.