netdev - RE: [Xen-devel] [PATCH net v2 1/3] xen-netback: remove pointless clause from if statement

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Date:	Fri, 28 Mar 2014 10:35:58 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Paul Durrant' <Paul.Durrant@...rix.com>,
	Sander Eikelenboom <linux@...elenboom.it>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: RE: [Xen-devel] [PATCH net v2 1/3] xen-netback: remove pointless
 clause from if statement

From: Paul Durrant
> > A reasonable high estimate for the number of slots required for a specific
> > message is 'frag_count + total_size/4096'.
> > So if that are that many slots free it is definitely ok to add the message.
> >
> 
> Hmm, that may work. By total_size, I assume you mean skb->len, so that calculation is based on an
> overhead of 1 non-optimally packed slot per frag. There'd still need to be a +1 for the GSO 'extra'
> though.

Except I meant '2 * frag_count + size/4096' :-(

You have to assume that every fragment starts at n*4096-1 (so need
at least two slots). A third slot is only needed for fragments
longer that 1+4096+2 - but an extra one is needed for every
4096 bytes after that.

	David



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