netdev - Re: [PATCH net-next] tcp: use an RB tree for ooo receive queue

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Date:   Wed, 7 Sep 2016 15:26:37 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Yaogong Wang <wygivan@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: Re: [PATCH net-next] tcp: use an RB tree for ooo receive queue

On Wed, 07 Sep 2016 14:49:28 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:

> From: Yaogong Wang <wygivan@...gle.com>
> 
> Over the years, TCP BDP has increased by several orders of magnitude,
> and some people are considering to reach the 2 Gbytes limit.
> 
> Even with current window scale limit of 14, ~1 Gbytes maps to ~740,000
> MSS.
>     
> In presence of packet losses (or reorders), TCP stores incoming packets
> into an out of order queue, and number of skbs sitting there waiting for
> the missing packets to be received can be in the 10^5 range.
> 
> Most packets are appended to the tail of this queue, and when
> packets can finally be transferred to receive queue, we scan the queue
> from its head.
> 
> However, in presence of heavy losses, we might have to find an arbitrary
> point in this queue, involving a linear scan for every incoming packet,
> throwing away cpu caches.
> 
> This patch converts it to a RB tree, to get bounded latencies.
> 
> Yaogong wrote a preliminary patch about 2 years ago.
> Eric did the rebase, added ofo_last_skb cache, polishing and tests.
> 
> Tested with network dropping between 1 and 10 % packets, with good
> success (about 30 % increase of throughput in stress tests)
> 
> Next step would be to also use an RB tree for the write queue at sender
> side ;)
> 
> Signed-off-by: Yaogong Wang <wygivan@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>

How much does this grow the size of tcp socket structure?