netdev - Re: TCP window auto-tuning sub-optimal in GRE tunnel

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Date:	Mon, 25 May 2015 18:44:44 -0400
From:	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: TCP window auto-tuning sub-optimal in GRE tunnel

On Mon, 2015-05-25 at 15:38 -0700, Eric Dumazet wrote:
> On Mon, 2015-05-25 at 18:22 -0400, John A. Sullivan III wrote:
> 
> > 2) Why do we still not negotiate the 16MB buffer that we get when we are
> > not using GRE?
> 
> What exact NIC handles receive side ?
> 
> If drivers allocate a full 4KB page to hold each frame,
> plus sk_buff overhead,
> then 32MB of kernel memory translates to 8MB of TCP window space.
> 
> 
> 
> 
Hi, Eric.  I'm not sure I understand the question or how to obtain the
information you've requested.  The receive side system has 48GB of RAM
but that does not sound like what you are requesting.

I suspect the behavior is a "protection mechanism", i.e., it is being
calculated for good reason.  When I set the buffer to 16MB manually in
nuttcp, performance degraded so I assume I was overrunning something.  I
am still downloading the traces.

But I'm still mystified by why this only affects GRE traffic.  Thanks -
John

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