netdev - Re: [PATCH net-next] tcp: prepare skbs for better sack shifting

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Message-Id: <20160917.100527.590702360910250037.davem@davemloft.net>
Date:   Sat, 17 Sep 2016 10:05:27 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org, ycheng@...gle.com
Subject: Re: [PATCH net-next] tcp: prepare skbs for better sack shifting

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 15 Sep 2016 09:33:02 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> With large BDP TCP flows and lossy networks, it is very important
> to keep a low number of skbs in the write queue.
> 
> RACK and SACK processing can perform a linear scan of it.
> 
> We should avoid putting any payload in skb->head, so that SACK
> shifting can be done if needed.
> 
> With this patch, we allow to pack ~0.5 MB per skb instead of
> the 64KB initially cooked at tcp_sendmsg() time.
> 
> This gives a reduction of number of skbs in write queue by eight.
> tcp_rack_detect_loss() likes this.
> 
> We still allow payload in skb->head for first skb put in the queue,
> to not impact RPC workloads.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>

Applied.