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Re: [Xen-devel] [RFC PATCH] page_alloc: use first half of higher order chunks when halving

On Wed, Mar 26, 2014 at 11:45:50AM +0000, Andrew Cooper wrote:
> On 26/03/14 11:41, Matt Wilson wrote:
> > On Wed, Mar 26, 2014 at 11:13:49AM +0000, Ian Campbell wrote:
> >> On Wed, 2014-03-26 at 12:48 +0200, Matt Wilson wrote:
> >>> I imagine that some users will not want to run dom0 under an IOMMU.
> >>
> >> Then they have chosen that (for whatever reason) over performance,
> >> right?
> >
> > IOMMU is not free, so I imagine that some users would actually be
> > choosing to avoid it specifically for performance reasons.
> >
> > --msw
> True, but if people are actually looking for performance, they will be
> turning on features like GRO and more generically scatter/gather which
> results in drivers using single page DMA mappings at a time, which
> completely bypass the bouncing in the swiotlb.

Alas, GRO isn't without its own problems (at least historically).


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