Re: [Xen-users] Balloon Driver Documentation/Description

[Timo Benk <timo.benk@xxxxxx>]

Mark Williamson wrote:
>> i'm looking for some documentation on how the balloon driver works
>> internally.
>>
>> Can anybody give me a hint were i can find some good doku?
> 
> Basically, the balloon driver allocates memory using standard kernel 
> allocation functions, then gives it back to Xen.  The memory is no longer 
> available to use in the guest at this point, and the fact that it was 
> allocated by the balloon driver prevents the rest of the kernel from trying 
> to use it.
> 
> When the balloon driver reclaims memory, it gets it back from Xen and then 
> does a "free", allowing the kernel memory allocator to give it to other 
> drivers for normal use.
> 

That is my understanding of the balloon driver. What i do not understand
is, if the balloon driver consists just of a frontend part in DomU that
is allocating some memory, and a backend driver in Dom0, that gives that
memory back, why is the output of the free command correct?

What i want to say is the following:

xendom1:~ # free
             total       used       free
Mem:        786432     117104     669328

xendom1:~ # echo -n "536870912" > /proc/xen/balloon

xendom1:~ # free
             total       used       free
Mem:        524288     116664     407624

Why is the total amount of RAM also changing. If the balloon driver just
allocates the memory, the used value should raise, but the total value
should stay.

I am not a kernel hacker, but i think i need to take a look into the
code to understand that.

Greetings,
-timo

-- 
Timo Benk - Jabber ID: fry@xxxxxxxxxx - ICQ ID: #241877854
PGP Public Key: http://m28s01.vlinux.de/timo_benk_gpg_key.asc


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