netdev - Re: [RFC] cubic: backoff after slow start

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Date:	Tue, 7 Aug 2007 20:57:50 -0400
From:	"Injong Rhee" <rhee@...u.edu>
To:	"Stephen Hemminger" <shemminger@...ux-foundation.org>,
	"Injong Rhee" <rhee@....ncsu.edu>, "Sangtae Ha" <sha2@...u.edu>
Cc:	<netdev@...r.kernel.org>
Subject: Re: [RFC] cubic: backoff after slow start

Hi Stephen,
We have been working on slow start and we have a nice solution for this. We 
will send you a patch and test results.
Thanks
Injong


----- Original Message ----- 
From: "Stephen Hemminger" <shemminger@...ux-foundation.org>
To: "Injong Rhee" <rhee@....ncsu.edu>; "Sangtae Ha" <sha2@...u.edu>
Cc: <netdev@...r.kernel.org>
Sent: Tuesday, August 07, 2007 2:37 PM
Subject: [RFC] cubic: backoff after slow start


> CUBIC takes several unnecessary iterations to converge out of slow start. 
> This
> is most noticable over a link where the bottleneck queue size is much 
> larger than BDP,
> and the sender has to "fill the pipe" in slow start before the first loss. 
> Typical
> consumer broadband links seem to have large (up to 2secs) of queue that 
> needs
> to get filled before the first loss.
>
> A possible fix is to use a beta of .5 (same as original TCP) when leaving
> slow start.  Originally, the Linux version didn't do slow start so it 
> probably
> never was observed.
>
> --- a/net/ipv4/tcp_cubic.c 2007-08-02 12:16:22.000000000 +0100
> +++ b/net/ipv4/tcp_cubic.c 2007-08-03 15:57:12.000000000 +0100
> @@ -289,7 +289,11 @@ static u32 bictcp_recalc_ssthresh(struct
>
>  ca->loss_cwnd = tp->snd_cwnd;
>
> - return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
> + /* Initial backoff when leaving slow start */
> + if (tp->snd_ssthresh == 0x7fffffff)
> + return max(tp->snd_cwnd >> 1U, 2U);
> + else
> + return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
> }
>
> static u32 bictcp_undo_cwnd(struct sock *sk)
> 






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