netdev - Re: [PATCH net-next] tcp: limit GSO packets to half cwnd

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Date:	Thu, 13 Nov 2014 13:24:42 -0500
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>
Subject: Re: [PATCH net-next] tcp: limit GSO packets to half cwnd

On Thu, Nov 13, 2014 at 12:45 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> In DC world, GSO packets initially cooked by tcp_sendmsg() are usually
> big, as sk_pacing_rate is high.
>
> When network is congested, cwnd can be smaller than the GSO packets
> found in socket write queue. tcp_write_xmit() splits GSO packets
> using the available cwnd, and we end up sending a single GSO packet,
> consuming all available cwnd.
>
> With GRO aggregation on the receiver, we might handle a single GRO
> packet, sending back a single ACK.
>
> 1) This single ACK might be lost
>    TLP or RTO are forced to attempt a retransmit.
> 2) This ACK releases a full cwnd, sender sends another big GSO packet,
>    in a ping pong mode.
>
> This behavior does not fill the pipes in the best way, because of
> scheduling artifacts.
>
> Make sure we always have at least two GSO packets in flight.
>
> This allows us to safely increase GRO efficiency without risking
> spurious retransmits.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/ipv4/tcp_output.c |   12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Eric!

neal
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