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

[James Harper <james.harper@xxxxxxxxxxxxxxxx>]

> > 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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