netdev - Re: [RFC PATCH net-next 0/3] sctp: add GSO support

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Date:	Thu, 28 Jan 2016 13:53:23 -0200
From:	"'Marcelo Ricardo Leitner'" <marcelo.leitner@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Neil Horman <nhorman@...driver.com>,
	Vlad Yasevich <vyasevich@...il.com>,
	David Miller <davem@...emloft.net>,
	"brouer@...hat.com" <brouer@...hat.com>,
	"alexander.duyck@...il.com" <alexander.duyck@...il.com>,
	"alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>,
	"borkmann@...earbox.net" <borkmann@...earbox.net>,
	"marek@...udflare.com" <marek@...udflare.com>,
	"hannes@...essinduktion.org" <hannes@...essinduktion.org>,
	"fw@...len.de" <fw@...len.de>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"john.r.fastabend@...el.com" <john.r.fastabend@...el.com>,
	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>
Subject: Re: [RFC PATCH net-next 0/3] sctp: add GSO support

On Thu, Jan 28, 2016 at 01:51:02PM +0000, David Laight wrote:
> From: Marcelo Ricardo Leitner
> > Sent: 27 January 2016 17:07
> > This patchset is merely a RFC for the moment. There are some
> > controversial points that I'd like to discuss before actually proposing
> > the patches.
> 
> You also need to look at how a 'user' can actually get SCTP to
> merge data chunks in the first place.
> 
> With Nagle disabled (and it probably has to be since the data flow
> is unlikely to be 'command-response' or 'unidirectional bulk')
> it is currently almost impossible to get more than one chunk
> into an ethernet frame.
> 
> Support for MSG_MORE would help.
> 
> Given the current implementation you can get almost the required
> behaviour by turning nagle off and on repeatedly.

That's pretty much expected, I think. Without Nagle, if bandwidth and
cwnd allow, segment will be sent. GSO by itself shouldn't cause a
buffering to protect from that.

If something causes a bottleneck, tx may get queue up.  Like if I do a
stress test in my system, generally receiver side is slower than sender,
so I end up having tx buffers pretty easily. It mimics bandwidth
restrictions.

There is also the case of sending large data chunks, where
sctp_sendmsg() will segment it into smaller chunks already.

But yes, agreed, MSG_MORE is at least a welcomed compliment here,
specially for applications generating a train of chunks. Will put that in
my ToDo here, thanks.

> I did wonder whether the queued data could actually be picked up
> be a Heartbeat chunk that is probing a different remote address
> (which would be bad news).

I don't follow. You mean if a heartbeat may get stuck in queue or if
sending of a heartbeat can end up carrying additional data by accident?

  Marcelo