Re: [Xen-devel] HVM guest max memory allocation

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Jul 30, 2015 at 06:21:06AM +0000, Hao, Xudong wrote:
> > -----Original Message-----
> > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > Sent: Friday, July 24, 2015 3:29 PM
> > To: Hao, Xudong
> > Cc: xen-devel@xxxxxxxxxxxxx; Wu, Feng; wei.liu2@xxxxxxxxxx
> > Subject: Re: [Xen-devel] HVM guest max memory allocation
> > 
> > On Fri, Jul 24, 2015 at 06:04:53AM +0000, Hao, Xudong wrote:
> > > Hi,
> > >
> > > When creating HVM guest(no balloon driver), what's the max memory we
> > > could set? We can get the current system free memory by "xl info", but
> > > when configure the free memory to a HVM guest, it fail to boot up.
> > > Does Xen allocate additional memory when do VM creating? How many the
> > > additional memory burning per HVM by Xen?
> > 
> > Historically libxl will add some slack on top of the memory you ask for.
> > 
> > The extra memory for HVM at the moment is 2 MB. That means if you ask for
> > 1024 MB for guest, libxl bumps that to 1026.
> > 
> 
> Wei, 
> 
> Thanks for your quick response, sorry I reply later as this mail missed by my 
> outlook... 
> OK, 2MB for tool stack per HVM.
> 
> > You also need to spare some memory for Xen itself to allocate internal
> > structures. I would say try to reduce the number for a few MBs.
> > 
> 
> Does different HVM configure get different memory consume of Xen itself? Can 
> we calculate an accurate boundary memory for HVM?

That would be hard I think, because when you have different features
enabled, Xen allocates different structures to keep track of guest
state.  

It is like a OS keeping track of the state of a process. It's hard to
tell in Linux how many kernel objects are allocated for a particular
process, no? It's also hard to tell how many tricks (which consumes
memory) Linux has done to make things run fast.

The closely thing I can think of is the debug key H and m. With those you
know how much actual memory is used. And, *if* Xen almost allocates all
structures up front you can have a relatively close approximation of
memory consumption.

George (CC'ed) might have some more ideas.

Wei.

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