netdev - Re: [PATCH net-next] tcp-tso: do not split TSO packets at retransmit time

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Date:	Tue, 19 Apr 2016 17:49:50 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	edumazet@...gle.com, netdev@...r.kernel.org, ncardwell@...gle.com,
	ycheng@...gle.com
Subject: Re: [PATCH net-next] tcp-tso: do not split TSO packets at
 retransmit time

On Tue, 2016-04-19 at 20:36 -0400, David Miller wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Mon, 18 Apr 2016 13:56:12 -0700
> 
> > 1 % packet losses are common today, and at 100Gbit speeds, this
> > translates to ~80,000 losses per second. If we are unlucky and
> > first MSS of a 45-MSS TSO is lost, we are cooking 44 MSS segments
> > at rtx instead of a single 44-MSS TSO packet.
> 
> I'm having trouble understanding this.
> 
> If the first mss is lost, then we simply chop the 45 MSS TSO skb into
> two pieces.  The first piece is a 1 MSS chunk for the retransmit, and
> the second piece is remaining 44 MSS TSO skb.
> 
> I am pretty sure that is what the current stack does, and regardless
> it is certainly what I intended it to do all those years ago when I
> wrote this code. :-)
> 
> The only case where I can see this patch helping is when we have to
> retransmit multi-mss chunks.  And yes indeed, it might be a useful
> optimization to TSO those frames rather than sending them one MSS at a
> time.

Yeah, it looks like I got the changelog wrong. We definitely see these
1-MSS splits during retransmits all the time, and we had to change the
sch_fq flow_limit from 100 to 1000 packets to cope with that. (TCP Small
Queues does not guard TCP from sending hundred of rtx at the same time)