netdev - Re: [PATCH net-next] tcp: speedup tcp_fixup

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Date:	Thu, 16 May 2013 11:23:03 -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>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next] tcp: speedup tcp_fixup_rcvbuf()

On Thu, May 16, 2013 at 1:25 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> tcp_fixup_rcvbuf() contains a loop to estimate initial socket
> rcv space needed for a given mss. With large MTU (like 64K on lo),
> we can loop ~500 times and consume a lot of cpu cycles.
>
> perf top of 200 concurrent netperf -t TCP_CRR
>
> 5.62%  netperf  [kernel.kallsyms]  [k] tcp_init_buffer_space
> 1.71%  netperf  [kernel.kallsyms]  [k] _raw_spin_lock
> 1.55%  netperf  [kernel.kallsyms]  [k] kmem_cache_free
> 1.51%  netperf  [kernel.kallsyms]  [k] tcp_transmit_skb
> 1.50%  netperf  [kernel.kallsyms]  [k] tcp_ack
>
> Lets use a 100% factor, and remove the loop.
>
> 100% is needed anyway for tcp_adv_win_scale=1
> default value, and is also the maximum factor.
>
> Refs: commit b49960a05e32
>       ("tcp: change tcp_adv_win_scale and tcp_rmem[2]")
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> ---
>  net/ipv4/tcp_input.c |    4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)

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

neal
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