netdev - RE: [PATCH net-next 1/8] tipc: decrease connection flow control window

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Date:	Fri, 9 May 2014 16:35:06 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Jon Maloy' <maloy@...jonn.com>,
	"'erik.hugne@...csson.com'" <erik.hugne@...csson.com>
CC:	'Jon Maloy' <jon.maloy@...csson.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"ying.xue@...driver.com" <ying.xue@...driver.com>,
	"tipc-discussion@...ts.sourceforge.net" 
	<tipc-discussion@...ts.sourceforge.net>
Subject: RE: [PATCH net-next 1/8] tipc: decrease connection flow control
 window

From: Jon Maloy
> On 05/09/2014 10:59 AM, David Laight wrote:
> > From: erik.hugne@...csson.
> >> On Fri, May 09, 2014 at 01:30:43PM +0000, David Laight wrote:
> >>> Sounds a bit like a badly designed protocol to me...
> >> Well, i don't like this either but the current message-oriented flowcontrol
> >> will eventually be replaced with a byte-based one. Right now we're trying
> >> to find a way to adapt a message-oriented flow control to per-socket buffer
> >> constraints, without breaking protocol compatibility.
> > I wasn't thinking of the byte/message flow control problems.
> > More of only requesting acks every 512 messages
> 
> Acks are not requested, they are sent out unsolicited at fix
> intervals, controlled by the reading capacity of the receiving
> process.

Requested by the received side...

> The fundamental problem we have is that we acknowledge
> messages, without considering their size, so a sender won't
> stop (there is no active stop message from the receiver) until
> he has sent 512 messages and not received an ack, which in
> the worst case amounts to 512 x truesize(64KB) = 67 MB
> outstanding data. This is what the receiving socket must
> be able to absorb, in order to avoid broken connections.

Even 512 x 64kB is a lot.
I thought that a 64k skb would only have a truesize of 72k (with 4k pages).
I'm also guessing that an ethernet controller that can do txp rx offloading
might have to allocate 64k skb - so a single 1500 byte ethernet frame
is likely to be in an skb with a truesize of 72k.
I'm not sure if this affects your 128k truesize though.

(Pasted from below)
> TIPC connections always run over a reliable media, - its own link
> layer. This means that the connection layer does not have any
> form of sequence numbering or retransmission, -it just assumes
> that everything arrives in order and without losses, as long
> as the link layer doesn't tell it otherwise.

Hmmm... If multiple connections are multiplexed over a single
link you are completely stuffed!

	David



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