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Re: [Xen-devel] Shadow Page Tables in Xen

To: Gianluca Guida <glguida@xxxxxxxxx>
Subject: Re: [Xen-devel] Shadow Page Tables in Xen
From: priya sehgal <priyagps@xxxxxxxxxxx>
Date: Wed, 22 Apr 2009 04:07:02 +0530 (IST)
Cc: gianluca.guida@xxxxxxxxxxxxx, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Tim Deegan <Tim.Deegan@xxxxxxxxxx>
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> > We have a course project, in which we have to improve
> the performance of live migration for HVM guests. It seems
> that to support live migration, all the page table entries
> in the shadow page table are marked as write protected, so
> as to know which pages are dirtied and to be sent to the
> other machine. Since, there will be many page faults leading
> to performance degradation, we want to reduce these page
> faults. In our course project, we are supposed to form
> groups of pages and if any page in the group hits the page
> fault (due to write-protection), we mark all the pages in
> the group as RW. This way we can reduce the page faults.
> >
> Have you actually measured this? I think that the major
> cause of page
> faults and VM slowdown is -- rather than page faults on
> write access
> -- the fact that we blow the shadow pagetables away
> everytime we clean
> the dirty bitmap, and this requires a long operation to
> remove from
> top to bottom all reference counts and reconstructing later
> the shadow
> pagetables on the next memory accesses.

We have not measured this, but we will benchmark it after making the changes. 
Since the number of page faults will reduce by a factor of "n",
where "n" is the size of the page group, it should help speed the VM. If n is 
large enough, say 1000
contiguous pages and the workload is such that it dirties consecutive pages, it 
should help in improving performance.  For very small values of "n" it might 
not help that much. 



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