[Ukfreebsd] FreeBSD on Xen {was Re: Recap: FreeBSD developer summit in Cambridge in August}

Robert Watson rwatson at FreeBSD.org
Mon Sep 22 14:13:46 BST 2008

On Mon, 22 Sep 2008, Progga wrote:

> On Mon, Sep 22, 2008 at 10:08:50AM +0100, Sevan / Venture37 wrote:
>> Xen DomU support, how far off are we from having a EC2 image for testing???
> Don't know about EC2, but my FreeBSD 7.0 VPS from rootbsd.net runs on Xen. 
> Perhaps they have patched it themselves.  This one is slower than the 
> previous Jail-based one though :-(

I can't speak to what they're doing in their configuration, but it is 
expected, generally, that Xen be slower than OS-centric virtualization 
techniques.  With Jail, you share a single kernel and scheduler and use only 
two rings, but with Xen you have multiple kernel instances, multiple 
schedulers, and three rings in use, so there's a lot more overhead, less 
efficient use of memory and caches, and a lot less transparency between layers 
that allow context-sensitive optimizations and scheduling.

This isn't to say that Xen is a bad idea, it's just a significantly different 
technical solution to the virtualization problem than Jail, and has its 
trade-offs.  The good sides are that you can have multiple VMs with different 
kernel versions, stronger isolation between them in terms of security and 
resource allocation, etc.  But Xen being "slower" is generally expected 
because a lot more work is being done to give the appearance of multiple 

Robert N M Watson
Computer Laboratory
University of Cambridge

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