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

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Message-Id: <20160419.203656.505846026424405432.davem@davemloft.net>
Date:	Tue, 19 Apr 2016 20:36:56 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	edumazet@...gle.com
Cc:	netdev@...r.kernel.org, ncardwell@...gle.com, ycheng@...gle.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH net-next] tcp-tso: do not split TSO packets at
 retransmit time

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.