netdev - Re: [RFC] [TCP 1/3] tcp: Add MSG_NEW_PACKET flag to indicate preferable packet boundaries

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Date:	Fri, 29 Jun 2012 17:38:55 +0200
From:	Andreas Gruenbacher <agruen@...bit.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC] [TCP 1/3] tcp: Add MSG_NEW_PACKET flag to indicate
 preferable packet boundaries

On Fri, 2012-06-29 at 17:11 +0200, Eric Dumazet wrote:
> On Fri, 2012-06-29 at 16:54 +0200, Andreas Gruenbacher wrote:
> > The MSG_NEW_PACKET flag indicates to sendmsg / sendpage that the message or
> > page should be put into a new packet even when there is still room left in the
> > previous packet.
> > 
> > In the tcp protocol, messages which are not sent immediately are queued.  When
> > more data is sent, it will be added to the last segment in that queue until
> > that segment is "full" whenever possible; only then is a new segment added.
> > Right now, there is no way to indicate when tcp should start a new segment.
> > The new flag allows to control that.
> > 
> > Signed-off-by: Andreas Gruenbacher <agruen@...bit.com>
> > ---
> 
> I don't understand how maintaining any message boundaries at sender can
> prevent any middlebox or the receiver to coalesce frames to any
> boundaries it prefers ?

The primary use case is fast Gigabit (10 or more) Ethernet connections
with jumbo frames and switches that support them.  There, frames will go
through unchanged and you can zero-copy receive all the time.

Not sure how well the approach scales to other kinds of connections; it
may work often enough to be worth it.  When things get distorted between
the sender and the receiver and tcp_recvbio() fails, the data can still
be copied out of the socket as before.

Andreas

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