Re: [Xen-devel] question on balloon driver

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I'm trying to get into the internals of xen and especially the balloon
> driver. During the time I dealt with that, several questions came up:

Excellent!  The more people who know about this stuff (and preferably hack on 
it ;-) the better.

> * Does (and if yes, how) the balloon driver in an guest OS have to be
> activated?

It's purely manual right now: you set a target memory allocation and the 
balloon driver will try and resize the guest to fit.

The target is set by an interface in /proc, or by a message on the Xen control 
interface (soon to be replaced by a value in the xenstore registry).

> * Does the automatic release of memory from a guest OS to Xen already
> work?

Nope, we'd like some kind of daemon to run in dom0 and balloon guests 
according to their need.  This should probably include some activity 
statistics reported by the guests, so the daemon can figure out which needs 
more memory.  Swap statistics might give a good idea of which guests needed 
memory.

> * Is the limit of 100 Guest OSs (as mentioned on
> http://en.wikipedia.org/wiki/Xen_%28virtual_machine_monitor%29 ) really
> hardcoded somewhere or is it more a practical value (or does the limit
> not exist)?

You should be able to start several hundred guests before hitting any hard 
coded limits (e.g. number of event channels  - I'm told there's a plan to 
remove this restriction).  Before you get to that stage, most systems will 
run out of hardware oomph to run so many guests *and* still do something 
useful.  I think Ian started somewhere over 100 guests one time and IIRC he 
ran out of disk images before hitting any limits :-)

Any limits will likely be addressed, since people are a) starting to use very 
large machines with Xen (Unisys are running it on 32 CPU boxes) and b) using 
cunning tricks to get extremely large numbers of guests running (I've heard 
"thousands" mentioned).

Cheers,
Mark

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