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

To: priya sehgal <priyagps@xxxxxxxxxxx>
Subject: Re: [Xen-devel] Shadow Page Tables in Xen
From: Gianluca Guida <glguida@xxxxxxxxx>
Date: Tue, 21 Apr 2009 16:59:00 +0200
Cc: gianluca.guida@xxxxxxxxxxxxx, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Tim Deegan <Tim.Deegan@xxxxxxxxxx>
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On Tue, Apr 21, 2009 at 1:37 PM, priya sehgal <priyagps@xxxxxxxxxxx> wrote:
> Thanks Tim.
>> Why are you interested?  Do you have a project in
>> mind?
> 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.


It was a type of people I did not know, I found them very strange and
they did not inspire confidence at all. Later I learned that I had been
introduced to electronic engineers.
                                                  E. W. Dijkstra

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