netdev - Re: [RFC net-next 0/6] tcp: remove prequeue and header prediction

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Message-Id: <20170729.152244.43564304647234406.davem@davemloft.net>
Date:   Sat, 29 Jul 2017 15:22:44 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     fw@...len.de
Cc:     netdev@...r.kernel.org, ycheng@...gle.com, ncardwell@...gle.com,
        edumazet@...gle.com, soheil@...gle.com, weiwan@...gle.com,
        brakmo@...com
Subject: Re: [RFC net-next 0/6] tcp: remove prequeue and header prediction

From: Florian Westphal <fw@...len.de>
Date: Fri, 28 Jul 2017 01:31:11 +0200

> This RFC removes tcp prequeueing and header prediction support.
> 
> After a hallway discussion with Eric Dumazet some
> maybe-not-so-useful-anymore TCP stack features came up, HP and
> Prequeue among these.
> 
> So this RFC proposes to axe both.
> 
> In brief, TCP prequeue assumes a single-process-blocking-read
> design, which is not that common anymore, and the most frequently
> used high-performance networking program that does this is netperf :)
> 
> With more commong (e)poll designs, prequeue doesn't work.
> 
> The idea behind prequeueing isn't so bad in itself; it moves
> part of tcp processing -- including ack processing (including
> retransmit queue processing) into process context.
> However, removing it would not just avoid some code, for most
> programs it elimiates dead code.
> 
> As processing then always occurs in BH context, it would allow us
> to experiment e.g. with bulk-freeing of skb heads when a packet acks
> data on the retransmit queue.
> 
> Header prediction is also less useful nowadays.
> For packet trains, GRO will aggregate packets so we do not get
> a per-packet benefit.
> Header prediction will also break down with light packet loss due to SACK.
> 
> So, In short: What do others think?

I have no objections to any of this. :)