netdev - RE: [Xen-devel] [PATCH 5/6] xen-netback: coalesce slots before copying

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Date:	Tue, 26 Mar 2013 11:29:26 +0000
From:	James Harper <james.harper@...digoit.com.au>
To:	Paul Durrant <Paul.Durrant@...rix.com>,
	Wei Liu <wei.liu2@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	Ian Campbell <Ian.Campbell@...rix.com>, Wei Liu <liuw@...w.name>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"annie.li@...cle.com" <annie.li@...cle.com>
Subject: RE: [Xen-devel] [PATCH 5/6] xen-netback: coalesce slots before
 copying

> > As stated previously, I've observed windows issuing staggering numbers of
> > buffers to NDIS miniport drivers, so you will need to coalesce in a windows
> > driver anyway. I'm not sure what the break even point is but I think it's safe
> > to say that in the choice between using 1000 (worst case) ring slots (with
> > the
> > resulting mapping overheads) and coalescing in the frontend, coalescing is
> > going to be the better option.
> >
> 
> Oh quite, if the backend is mapping and not copying then coalescing in the
> frontend is the right way to go. I guess coalescing once the frag count
> reaches a full ring count is probably necessary (since we can't push a partial
> packet) but it would be nice not to have to do it if the backend is going to
> copy anyway.
> 

For a 9k packet with 100 frags (not a common case, but an example), what is the cost of mapping those 100 frags into the backend vs coalescing to three pages in the frontend and mapping those?

I may be misremembering but wasn't there a patch floating around for persistent mapping to avoid some of this overhead? (not applicable here but I thought it meant that the cost wasn't insignificant)

James

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