Re: [Xen-devel] PoD, 4.2, and current/maximum reservation

[James Harper <james.harper@xxxxxxxxxxxxxxxx>]

> 
> On Mon, Feb 11, 2013 at 12:03 AM, James Harper
> <james.harper@xxxxxxxxxxxxxxxx> wrote:
> 
>       A user has pointed out a problem with GPLPV under Xen 4.2 when
> using PoD. I'm using the difference between
> XENMEM_maximum_reservation and XENMEM_current_reservation to tell
> me how much I should balloon down to account for PoD, but when the user
> has ballooned down to 1G (from 4Gb or 8GB), GPLPV logs the following:
> 
>       13005008825593: XenPCI     XENMEM_maximum_reservation =
> 262400
>       13005008825593: XenPCI     XENMEM_current_reservation = 262136
>       13005008825609: XenPCI     Trying to give 1056 KB (1 MB) to Xen
> 
>       What is the correct way to tell how much PoD memory there is under
> 4.2? Am I doing it wrong?
> 
>       I balloon down as early as possible (before xenbus starts) to avoid
> windows going over its limit so I'm hoping I can determine the size of PoD
> memory just via hypercalls.
> 
> 
> 
> You shouldn't have to know anything specifically about PoD -- you should just
> look at the balloon target for the guest written in xenstore.  The idea was as
> much as possible for the toolstack and Xen to work together to make it
> transparent to the balloon driver, in part because we expected to be running
> legacy drivers.  The Citrix PV drivers don't do anything special wrt PoD
> memory.  (Paul, please correct me if I'm wrong.)

So I should just balloon down to the current_reservation figure right?

> WRT timing and xenbus, a couple of things:
> 
> * Windows does a scrub of all memory at start-of-day.  Especially on multiple-
> vcpu systems, we were unable to start the balloon process early enough to
> win the race against the scrubber, so we had to have ways of "reclaiming"
> zeroed pages for the PoD pool.  What this means is that it's not a matter of
> Windows *touching* memory, but of windows *dirtying* memory that will
> lead to a problem.
> 
> * So there is a minimum amount of memory Windows needs to be able to
> make it to the stage where the balloon driver can run.  When XenServer first
> implemented DMC, the team did extensive testing to determine minimum
> values above which Windows never crashed or hung, and (as I understand it)
> baked those into the xapi toolstack as a "seatbelt" to prevent users from
> setting the value too low.
> 
> Not sure if that helps in your particular case -- I think 1G was within the 
> limit,
> but I could be wrong.  Dave, any comments?
> 

I think I could go from 4GB down to 128MB reliably without crashing, although 
the resulting OS wasn't particularly usable :)

Thanks

James

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