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Re: [Xen-devel] [PATCH RFC 00/10] Xen balloon page compaction support

On Wed, Oct 15, 2014 at 05:54:24PM +0100, Andrew Cooper wrote:
> On 15/10/14 16:54, Wei Liu wrote:
> > Hi all
> >
> > This is a prototype to make Xen balloon driver work with balloon page
> > compaction. The goal is to reduce guest physical address space fragmentation
> > introduced by balloon pages. Having guest physical address space as 
> > contiguous
> > as possible is generally useful (because guest can have higher order 
> > pages), and
> > it should also help improve HVM performance (provided that guest kernel 
> > knows
> > how to use huge pages -- Linux has hugetlbfs and transparent huge page).
> After you have defragmented guest physical memory, does Linux use
> 2MB/1GB superpages more readily?

That's completely up to the guest. Having contiguous guest physical
address space is prerequisite.

> On what basis do you think this will improve HVM performance?  The HAP
> tables still remain fragmented after ballooning.

That's the other side of this problem and orthogonal to this patch
series. It should be fixed separately on the hypervisor side, presumably
with similar mechanism to coalesce HAP table in hypervisor.


> ~Andrew

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