Re: Xen ballooning interface

[Juergen Gross <jgross@suse.com>]

On 14/08/18 09:02, Jan Beulich wrote:
>>>> On 13.08.18 at 17:44, <jgross@suse.com> wrote:
>> On 13/08/18 17:29, Jan Beulich wrote:
>>>>>> On 13.08.18 at 16:20, <jgross@suse.com> wrote:
>>>> On 13/08/18 15:54, Jan Beulich wrote:
>>>>>>>> On 13.08.18 at 15:06, <jgross@suse.com> wrote:
>>>>>> Suggested new interface
>>>>>> -----------------------
>>>>>> Hypercalls, memory map(s) and ACPI tables should stay the same (for
>>>>>> compatibility reasons or because they are architectural interfaces).
>>>>>>
>>>>>> As the main confusion in the current interface is related to the
>>>>>> specification of the target memory size this part of the interface
>>>>>> should be changed: specifying the size of the ballooned area instead
>>>>>> is much clearer and will be the same for all guest types (no firmware
>>>>>> memory or magic additions involved).
>>>>>
>>>>> But isn't this backwards? The balloon size is a piece of information
>>>>> internal to the guest. Why should the outside world know or care?
>>>>
>>>> Instead of specifying an absolute value to reach you'd specify how much
>>>> memory the guest should stay below its maximum. I think this is a valid
>>>> approach.
>>>
>>> But with you vNUMA model there's no single such value, and nothing
>>> like a "maximum" (which would need to be per virtual node afaics).
>>
>> With vNUMA there is a current value of memory per node supplied by the
>> tools and a maximum per node can be caclulated the same way.
> 
> Can it? If so, I must be overlooking some accounting done
> somewhere. I'm only aware of a global maximum.

The tools set the vnuma information for the guest. How do they do this
without knowing the memory size per vnuma node?

> 
>> This results in a balloon size per node.
>>
>> There is still the option to let the guest adjust the per node balloon
>> sizes after reaching the final memory size or maybe during the process
>> of ballooning at a certain rate.
> 
> I'm probably increasingly confused: Shouldn't, for whichever value
> in xenstore, there be a firm determination of which single party is
> supposed to modify a value? Aiui the intention is for the (target)
> balloon size to be set by the tools.

Sorry if I wasn't clear enough here: the guest shouldn't rewrite the
target balloon size, but e.g. memory/vnode<n>/balloon-size.

> 
>>>>>> Any further thoughts on this?
>>>>>
>>>>> The other problem we've always had was that address information
>>>>> could not be conveyed to the driver. The worst example in the past
>>>>> was that 32-bit PV domains can't run on arbitrarily high underlying
>>>>> physical addresses, but of course there are other cases where
>>>>> memory below a certain boundary may be needed. The obvious
>>>>> problem with directly exposing address information through the
>>>>> interface is that for HVM guests machine addresses are meaningless.
>>>>> Hence I wonder whether a dedicated "balloon out this page if you
>>>>> can" mechanism would be something to consider.
>>>>
>>>> Isn't this a problem orthogonal to the one we are discussing here?
>>>
>>> Yes, but I think we shouldn't design a new interface without
>>> considering all current shortcomings.
>>
>> I don't think the suggested interface would make it harder to add a way
>> to request special pages to be preferred in the ballooning process.
> 
> Address and (virtual) node may conflict with one another. But I
> think we've meanwhile settled on the node value to only be a hint
> in a request.

I think so, yes.


Juergen


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