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

> -----Original Message-----
> From: James Harper [mailto:james.harper@xxxxxxxxxxxxxxxx]
> Sent: 26 March 2013 11:01
> 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
> > > Because the check is >= MAX_SKB_FRAGS originally and James Harper
> told
> > > me that "Windows stops counting on 20".
> > >
> >
> > For the Citrix PV drivers I lifted the #define of MAX_SKB_FRAGS from the
> > dom0 kernel (i.e. 18). If a packet coming from the stack has more than that
> > number of fragments then it's copied and coalesced. The value advertised
> > for TSO size is chosen such that a maximally sized TSO will always fit in 18
> > fragments after coalescing but (since this is Windows) the drivers don't
> trust
> > the stack to stick to that limit and will drop a packet if it won't fit.
> >
> > It seems reasonable that, since the backend is copying anyway, that it
> should
> > handle any fragment list coming from the frontend that it can. This would
> > allow the copy-and-coalesce code to be removed from the frontend (and
> the
> > double-copy avoided). If there is a maximum backend packet size though
> > then I think this needs to be advertised to the frontend. The backend
> should
> > clearly bin packets coming from the frontend that exceed that limit but
> > advertising that limit in xenstore allows the frontend to choose the right
> > maximum size to advertise to its stack, rather than having to make it based
> > on some historical value that actually has little meaning (in the absence of
> > grant mapping).
> >
> 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 


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