netdev - Re: [PATCH 3/3] tcp_cubic: fix low utilization of CUBIC with HyStart

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Date:	Sun, 13 Mar 2011 19:07:07 -0500
From:	Sangtae Ha <sangtae.ha@...il.com>
To:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
Cc:	Stephen Hemminger <stephen.hemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	Injong Rhee <rhee@...u.edu>,
	Bill Fink <billfink@...dspring.com>, netdev@...r.kernel.org,
	Sangtae Ha <sangtae.ha@...il.com>
Subject: Re: [PATCH 3/3] tcp_cubic: fix low utilization of CUBIC with HyStart

Hi Lucas,

Thank you for reporting this problem and the results below show what
happened with low resolution clocks.
The detailed testing setup and results at
http://netsrv.csc.ncsu.edu/wiki/index.php/HyStart_testing_between_Princeton_and_NCSU
I will add the results in long rtt paths soon.

Sangtae

On Sun, Mar 13, 2011 at 7:04 PM, Sangtae Ha <sangtae.ha@...il.com> wrote:
> HyStart sets the initial exit point of slow start.
> Suppose that HyStart exits at 0.5BDP in a BDP network and no history exists.
> If the BDP of a network is large, CUBIC's initial cwnd growth may be
> too conservative to utilize the link.
> CUBIC increases the cwnd 20% per RTT in this case.
>
> Signed-off-by: Sangtae Ha <sangtae.ha@...il.com>
> ---
>  net/ipv4/tcp_cubic.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
>
> diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
> index b758db1..a170778 100644
> --- a/net/ipv4/tcp_cubic.c
> +++ b/net/ipv4/tcp_cubic.c
> @@ -270,6 +270,15 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
>                ca->cnt = 100 * cwnd;              /* very small increment*/
>        }
>
> +       /*
> +        * The initial growth of cubic function may be too conservative
> +        * when the available bandwidth is still unknown.
> +        */
> +       if (ca->loss_cwnd == 0) {
> +               if (ca->cnt > 20)       /* increase cwnd 5% per RTT */
> +                       ca->cnt = 20;
> +       }
> +
>        /* TCP Friendly */
>        if (tcp_friendliness) {
>                u32 scale = beta_scale;
> --
> 1.7.0.4
>
>
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