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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: James Harper [mailto:james.harper@xxxxxxxxxxxxxxxx]
> Sent: 26 March 2013 11:29
> To: Paul Durrant; Wei Liu; David Vrabel
> Cc: Ian Campbell; Wei Liu; netdev@xxxxxxxxxxxxxxx;
> konrad.wilk@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; annie.li@xxxxxxxxxx
> 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)
> 

The current version of netback does not map, it always grant-copies.

  Paul

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