netdev - Re: [PATCH] tcp: change tcp_adv_win_scale and tcp

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Message-ID: <CADVnQy=bYTuuYcAN95q7sD-f1Zw9DyYMH7ZP=m3fdXLCX+TGvw@mail.gmail.com>
Date:	Wed, 2 May 2012 15:48:47 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] tcp: change tcp_adv_win_scale and tcp_rmem[2]

On Wed, May 2, 2012 at 8:28 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> tcp_adv_win_scale default value is 2, meaning we expect a good citizen
> skb to have skb->len / skb->truesize ratio of 75% (3/4)
>
> In 2.6 kernels we (mis)accounted for typical MSS=1460 frame :
> 1536 + 64 + 256 = 1856 'estimated truesize', and 1856 * 3/4 = 1392.
> So these skbs were considered as not bloated.
>
> With recent truesize fixes, a typical MSS=1460 frame truesize is now the
> more precise :
> 2048 + 256 = 2304. But 2304 * 3/4 = 1728.
> So these skb are not good citizen anymore, because 1460 < 1728
>
> (GRO can escape this problem because it build skbs with a too low
> truesize.)
>
> This also means tcp advertises a too optimistic window for a given
> allocated rcvspace : When receiving frames, sk_rmem_alloc can hit
> sk_rcvbuf limit and we call tcp_prune_queue()/tcp_collapse() too often,
> especially when application is slow to drain its receive queue or in
> case of losses (netperf is fast, scp is slow). This is a major latency
> source.
>
> We should adjust the len/truesize ratio to 50% instead of 75%
>
> This patch :
>
> 1) changes tcp_adv_win_scale default to 1 instead of 2
>
> 2) increase tcp_rmem[2] limit from 4MB to 6MB to take into account
> better truesize tracking and to allow autotuning tcp receive window to
> reach same value than before. Note that same amount of kernel memory is
> consumed compared to 2.6 kernels.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Tom Herbert <therbert@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>

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

neal
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