netdev - Re: LRO/GRO and libpcap packet reordering

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Date:	Thu, 14 Mar 2013 15:58:12 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: LRO/GRO and libpcap packet reordering

On Thu, Mar 14, 2013 at 3:54 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2013-03-14 at 15:45 -0700, Andy Lutomirski wrote:
>
>> Hmm.  What if the GRO flow hashing was something like hash(one
>> endpoint) ^ hash(the other endpoint)?  (NB: I don't really know what
>> I'm talking about.)
>
> There is no hashing in GRO, but a full flow match.
>
> What you describe has to do with multi queue capability of the NIC.
>
> Each flow is handled by a separate RX queue, therefore by different
> cpus. There is no guarantee packets are delivered 'in order' in this
> case, GRO being off or on.

This happens even when there's just one rx queue.  (I checked that
before scratching my head for a while.)

>
> Of course, if GRO/LRO is on, there is an additional delay to permit
> aggregation, so a big GRO/LRO packet might appear _after_ an ACK.
>
>

Right.  This is the part that I'm wondering about an efficient fix to
(i.e. flush GRO state for one flow whenever a packet for the reversed
flow is seen).


--Andy
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