Re: Question about LRO/GRO and TCP acknowledgements

[Eric Dumazet <eric.dumazet@gmail.com>]

Le dimanche 12 juin 2011 à 13:24 +0200, Joris van Rantwijk a écrit :
> On 2011-06-12, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > So your concern is more a Sender side implementation missing this
> > recommendation, not GRO per se...
> 
> Not really. The same RFC says:
>   Specifically, an ACK SHOULD be generated for at least every
>   second full-sized segment, ...
> 

Well, SHOULD is not MUST.


> I can see how the world may have been a better place if every sender
> implemented Appropriate Byte Counting and TCP receivers were allowed to
> send fewer ACKs. However, current reality is that ABC is optional,
> disabled by default in Linux, and receivers are recommended to send one
> ACK per two segments.
> 

ABC might be nice for stacks that use byte counters for cwnd. We use
segments.

> I suspect that GRO currently hurts throughput of isolated TCP
> connections. This is based on a purely theoretic argument. I may be
> wrong and I have absolutely no data to confirm my suspicion.
> 
> If you can point out the flaw in my reasoning, I would be greatly
> relieved. Until then, I remain concerned that there may be something
> wrong with GRO and TCP ACKs.

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.

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.