Re: [Xen-users] Few Questions / Application of Xen

[Javier Guerra <javier@xxxxxxxxxxx>]

On Tuesday 21 March 2006 7:49 am, Steffen Heil wrote:
> Hi
>
> > > You need to give every domU a certain amount of memory on startup.
> > > As far as I know, you cannot change this later.
> > >
> > > So you NEED to give every domain as much memory as it will
> > > ever need.
> >
> > Yeah that sucks (for my use) :(
> >
> > No way of doing a no limit thing in order to share the RAM ?
> > Like each domU can take as many RAM as it wants and so it
> > would automagically share the RAM load ?
>
> I am also somehow new to xen, so somebody correct me if I am wrong.
> But as fas as I understand:
>
> Every domU has it's own kernel which manages it's own memory.
> So every domU also has it's own drive cache.
>
> Most OSs tend to simly use all free memory for drive caches. So does
> Windows and so does linux. This makes sence for real hardware, since unused
> free memory is a waste of resources and using every available memory as
> cache speeds up the mashine extremely. However, if there are other systems
> that share the same memory, It might be better for one system to have less
> cache memory and therefor preventing another system from swaping. But to
> take control of this, every system would need to know about the memory
> usage of the other systems.
> That would be totally against system separation.

that's all correct, but there's a nice hack to get that RAM back: the balloon 
driver.

it's a kernel module that just sits there in a domU taking up a variable 
amount of RAM, releasing or reclaiming it as commanded by messages from dom0.

so, you would start up many domUs giving each a big RAM allocation, but the 
balloon driver eats most of it. when a given domU needs more RAM, the Xen 
hypervisor maps some physical pages to that domU, and the balloon driver 
shrinks by the same amount, making the RAM usable by the rest of the kernel.


-- 
Javier

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