netdev - Re: [RFC PATCH net-next] tcp: add NV congestion control

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Date:	Fri, 03 Jul 2015 07:00:23 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Lawrence Brakmo <brakmo@...com>
Cc:	netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [RFC PATCH net-next] tcp: add NV congestion control

On Thu, 2015-07-02 at 18:21 -0700, Lawrence Brakmo wrote:
> This is a request for comments.
> 
> TCP-NV (New Vegas) is a major update to TCP-Vegas. An earlier version of
> NV was presented at 2010's LPC (slides). It is a delayed based
> congestion avoidance for the data center. This version has been tested
> within a 10G rack where the HW RTTs are 20-50us.
> 
> A description of TCP-NV, including implementation and experimental
> results, can be found at:
> http://www.brakmo.org/networking/tcp-nv/TCPNV.html
> 
> The current version includes many module parameters to support
> experimentation with the parameters.
> 
> Signed-off-by: Lawrence Brakmo <lawrence@...kmo.org>
> ---
>  include/linux/skbuff.h     |   2 +-
>  include/linux/tcp.h        |   4 +
>  include/net/tcp.h          |   5 +-
>  net/ipv4/Kconfig           |  16 ++
>  net/ipv4/Makefile          |   1 +
>  net/ipv4/sysctl_net_ipv4.c |   9 +
>  net/ipv4/tcp_input.c       |   5 +
>  net/ipv4/tcp_nv.c          | 477 +++++++++++++++++++++++++++++++++++++++++++++
>  net/ipv4/tcp_output.c      |   4 +-
>  9 files changed, 520 insertions(+), 3 deletions(-)
>  create mode 100644 net/ipv4/tcp_nv.c
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index d6cdd6e..96a131d 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -547,7 +547,7 @@ struct sk_buff {
>  	 * want to keep them across layers you have to do a skb_clone()
>  	 * first. This is owned by whoever has the skb queued ATM.
>  	 */
> -	char			cb[48] __aligned(8);
> +	char			cb[52] __aligned(8);
>  

skb bloat alert.

This adds 8 bytes to cb[], and sk_buff, for no reason.

tcp_skb_cb is currently 44 bytes, so even if you add one u32, it should
not exceed cb[]


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