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

From: James Harper <james.harper@bendigoit.com.au>
To: Paul Durrant <Paul.Durrant@citrix.com>,
	Wei Liu <wei.liu2@citrix.com>,
	David Vrabel <david.vrabel@citrix.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>, Wei Liu <liuw@liuw.name>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"konrad.wilk@oracle.com" <konrad.wilk@oracle.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	"annie.li@oracle.com" <annie.li@oracle.com>
Subject: RE: [Xen-devel] [PATCH 5/6] xen-netback: coalesce slots before copying
Date: Tue, 26 Mar 2013 11:29:26 +0000	[thread overview]
Message-ID: <6035A0D088A63A46850C3988ED045A4B3880B6AE@BITCOM1.int.sbss.com.au> (raw)
In-Reply-To: <291EDFCB1E9E224A99088639C4762022013F7D8E0E06@LONPMAILBOX01.citrite.net>

> > 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