[Xen-devel] Re: PoD issue

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

PoD is not critical to balloon out guest memory.  You can boot with mem 
== maxmem and then balloon down afterwards just as you could before, 
without involving PoD.  (Or at least, you should be able to; if you 
can't then it's a bug.)  It's just that with PoD you can do something 
you've always wanted to do but never knew it: boot with 1GiB with the 
option of expanding up to 2GiB later. :-)
With the 54 megabyte difference: It's not like a GiB vs GB thing, is 
it?  (i.e., 2^30 vs 10^9?)  The difference between 1GiB (2^30) and 1 GB 
(10^9) is about 74 megs, or 18,000 pages.
I guess that is a weakness of PoD in general: we can't control the guest 
balloon driver, but we rely on it to have the same model of how to 
translate "target" into # pages in the balloon as the PoD code.
-George

Jan Beulich wrote:
> > > > George Dunlap <george.dunlap@xxxxxxxxxxxxx> 29.01.10 17:01 >>>
> > > >         
> > What seems likely to me is that Xen (setting the PoD target) and the 
> > balloon driver (allocating memory) have a different way of calculating 
> > the amount of guest memory.  So the balloon driver thinks it's done 
> > handing memory back to Xen when there are still more outstanding PoD 
> > entries than there are entries in the PoD memory pool.  What balloon 
> > driver are you using?
> >     
> The one from our forward ported 2.6.32.x tree. I would suppose there
> are no significant differences here to the one in 2.6.18, but I wonder
> how precise the totalram_pages value is that the driver (also in 2.6.18)
> uses to initialize bs.current_pages. Given that with PoD it is now crucial
> for the guest to balloon out enough memory, using an imprecise start
> value is not acceptable anymore. The question however is what more
> reliable data source one could use (given that any non-exported
> kernel object is out of question). And I wonder how this works reliably
> for others...
>
>   
> > Can you let me know max_mem, target, and what the 
> > balloon driver has reached before calling it quits?  (Although 13,000 
> > pages is an awful lot to be off by: 54 MB...)
> >     
> The balloon driver reports the expected state: target and allocation
> are 1G. But yes - how did I not pay attention to this - the balloon is
> *far* from being 1G in size (and in fact the difference is probably
> matching quite closely those 54M).
>
> Thanks a lot!
>
> Jan
>
>   

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