netdev - Re: [PATCH net-next] tcp: fix rcv_ssthresh regression for medium-sized initial packets

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Date:	Tue, 23 Oct 2012 17:57:23 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] tcp: fix rcv_ssthresh regression for
 medium-sized initial packets

On Tue, 2012-10-23 at 11:35 -0400, Neal Cardwell wrote:
> The recent skb truesize fixes caused a slight regression in that
> moderately-sized initial incoming packets (eg ~1000 bytes) can now
> lead to smaller rcv_ssthresh increments than in 2.6.
> 
> The mitigation b49960a05e32121d ("tcp: change tcp_adv_win_scale and
> tcp_rmem[2]") compensates for the truesize fixes by fixing the
> ultimate value to which receive window converges. But this commit did
> not mitigate the impact for the initial cwnd growth phase for
> connetions with moderately-sized initial incoming packets. Such
> connections still hit the __tcp_grow_window() code path, where the
> increment to rcv_sssthresh was by rcv_mss, and not the generally
> larger advmss.
> 
> We should not handicap this initial growth rate of the receive
> window. That way short connections are not penalized in cases where
> they wouldn't lead to receive buffer memory consumption issues.
> 
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> CC: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/ipv4/tcp_input.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 60cf836..e5981a3 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -316,7 +316,7 @@ static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb)
>  
>  	while (tp->rcv_ssthresh <= window) {
>  		if (truesize <= skb->len)
> -			return 2 * inet_csk(sk)->icsk_ack.rcv_mss;
> +			return 2 * tp->advmss;
>  
>  		truesize >>= 1;
>  		window >>= 1;

But this defeats whole point of having a small initial window and
increase it if packets are 'good citizens' ?

Are you sure we wont drop packets later because we hit sk->sk_rcvbuf
limit ?

This brings back the receiver initial window we discussed lately.

Because if we had an initial window of 32KB or 64KB instead of 10*1460,
I am pretty sure we would not have to change this code ?

Thanks


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