Re: [Xen-devel] Xen - Mosix cluster

["Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>]

One can imagine some uses of Mosix in XenLinux atop Xen.  Xen still gives 
isolated driver domains and the ability to migrate domains around when you 
want to take a machine down for maintenance.  These features might be useful 
even if you only have one Mosix VM per node.

I agree that most people will probably just have a straight Mosix cluster or 
use Xen just as a test environment.

> If, however, Xen were on top of the stack, we may be able to achive  both
> purposes.

This is the real problem - Xen is specifically designed to live under the 
operating system kernel and provide a hardware-like abstraction.  It's not 
really aware that processes exist - they're a guest OS concept.  Xen is not 
really aware of the abstractions within a domain...

I see where you're coming from, though - it'd be neat to pool resources more 
flexibly this way.

> From Xens point of view, it would just have a way
> big honker of a machine to work with.

Since Xen lives at the bottom of the stack, this isn't really possible - Xen 
itself is really intimately tied to the specific machine its working on.  In 
contrast, Mosix can provide users with the illusion of a big machine but it's 
intimitely tied into the Linux kernel.

I don't know so much about OpenSSI, how does that abstract things to the user?

> This would also allow a high demand 
> OS environment to grow past the single machine limit.  This might also help
> with issues of bringing nodes on and off line.

This leads to an interesting thought though - Xen does accurate resource 
accounting on what domains have used.  That's one of its strengths.  A cool 
idea (although not one that'd necessarily get done) that'd partially address 
your problem would be to plug Xend into Mosix's process migration mechanisms.

e.g. only allow a process migration to another node if the domain owner has 
paid enough and then keep track of the resource usage on the remote node *as 
well*, so that the total resource usage is known.  One could imagine creating 
XenLinux/Mosix domains on other nodes on-demand when the user's virtual 
machine wants to migrate a compute-intensive process.  (Domains running in a 
ramdisk only take a few hundred milliseconds to start, so this is quite 
feasible).

This way one could get some of the advantages you mention and retain the 
strong isolation, resource accounting, etc.

I'll have a think about that, since it does sound kinda cool ;-)

Mark


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