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

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Date:	Tue, 26 Mar 2013 11:24:12 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	James Harper <james.harper@...digoit.com.au>,
	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

> -----Original Message-----
> From: James Harper [mailto:james.harper@...digoit.com.au]
> Sent: 26 March 2013 11:01
> To: Paul Durrant; Wei Liu; David Vrabel
> Cc: Ian Campbell; Wei Liu; netdev@...r.kernel.org;
> konrad.wilk@...cle.com; xen-devel@...ts.xen.org; annie.li@...cle.com
> 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
> TSO
> > 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 anyway.

  Paul
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