netdev - Re: [PATCH net-next 0/3] tcp: improving RACK cpu performance

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Date:   Fri, 6 Oct 2017 09:34:48 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 0/3] tcp: improving RACK cpu performance

On Thu, Oct 5, 2017 at 9:26 PM, David Miller <davem@...emloft.net> wrote:
>
> From: Yuchung Cheng <ycheng@...gle.com>
> Date: Wed,  4 Oct 2017 12:59:57 -0700
>
> > This patch set improves the CPU consumption of the RACK TCP loss
> > recovery algorithm, in particular for high-speed networks. Currently,
> > for every ACK in recovery RACK can potentially iterate over all sent
> > packets in the write queue. On large BDP networks with non-trivial
> > losses the RACK write queue walk CPU usage becomes unreasonably high.
> >
> > This patch introduces a new queue in TCP that keeps only skbs sent and
> > not yet (s)acked or marked lost, in time order instead of sequence
> > order.  With that, RACK can examine this time-sorted list and only
> > check packets that were sent recently, within the reordering window,
> > per ACK. This is the fastest way without any write queue walks. The
> > number of skbs examined per ACK is reduced by orders of magnitude.
>
> That's a pretty risky way to implement the second SKB list.... but
Agreed. I really appreciate you accepting this change. We tried a few
alternatives but this is the most clean and fastest approach. BBR can
reach 8-10Gbps on 300ms RTT link so both RACK and SACK need to scale
in the era of hundreds MB BDP :-)

> you avoided making sk_buff larger so what can I say :-)
>
> Series applied, thank.