netdev - Re: Question about LRO/GRO and TCP acknowledgements

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Message-ID: <20110612213726.4d203a6e@konijn>
Date:	Sun, 12 Jun 2011 21:37:26 +0200
From:	Joris van Rantwijk <joris@...isvr.nl>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Question about LRO/GRO and TCP acknowledgements

On 2011-06-12, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Think of GRO being a receiver facility against stress/load, typically
> in datacenter.
> 
> Only when receiver is overloaded, GRO kicks in and can coalesce
> several frames before being handled in TCP stack in one run.

Ok, it now becomes clear to me that I have a different scenario in mind
than GRO was designed to handle. I'm interested in LRO as a method
to sustain 1 Gbit through a single TCP connection on a slow embedded
computer.

> If receiver is so loaded that more than 2 frames are coalesced in a
> NAPI run, it certainly helps to not allow sender to increase its cwnd
> more than one SMSS. We probably are right before packet drops anyway.

Right. So unlike TSO, GRO is not a transparent, generally applicable
performance improvement. It's more like a form of graceful degradation,
helping a server to sustain overall throughput when it is already
swamped in TCP traffic.

Thanks for your clarification. This has certainly solved some confusion
on my side.

Joris.
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