Re: Copy-on-write memory to allow many more xenU domains per machine

[Michael Vrable <mvrable@cs.ucsd.edu>]

On Tue, Oct 26, 2004 at 08:43:30PM +0100, Ian Pratt wrote:
> > "Memory Resource Management in VMware ESX Server" by Carl Waldspurger
> > has some numbers on memory sharing between VMs:
> >     http://www.usenix.org/events/osdi02/tech/waldspurger/waldspurger.pdf
> > (This paper is also cited in the Xen SOSP 2003 paper.)
> > 
> > Sharing between identical machines is good, though sharing between
> > dissimilar machines is not as high as I remembered (copy-on-write
> > sharing is able to save around 7-30% of memory in real-world testing).
> 
> As I recall, Carl's results were on Windows, which might explain
> why I was seeing rather less. Linux resident set sizes tend to be
> smaller, and there's generally rather more diversity in the
> install base (at least around here).

Both were tested in that paper; Windows was at the high end of the scale
and Linux at the low end:

Reproducing Figure 5 from the paper in slightly abbreviated form:

          Guests        % Shared        % Reclaimed
        10 x WinNT        42.9              32.9
         9 x Linux        29.2              18.7
         5 x Linux        10.0               7.2

"% Shared" is the fraction of pages which are shared; "% Reclaimed"
measures how much memory sharing the pages is able to save.

Three systems is still a limited test set.

--Michael Vrable


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