Re: On guest free page hinting and OOM

David Hildenbrand
2019-03-29T15:46:09Z

On 29.03.19 16:37, David Hildenbrand wrote:
> On 29.03.19 16:08, Michael S. Tsirkin wrote:
>> On Fri, Mar 29, 2019 at 03:24:24PM +0100, David Hildenbrand wrote:
>>>
>>> We had a very simple idea in mind: As long as a hinting request is
>>> pending, don't actually trigger any OOM activity, but wait for it to be
>>> processed. Can be done using simple atomic variable.
>>>
>>> This is a scenario that will only pop up when already pretty low on
>>> memory. And the main difference to ballooning is that we *know* we will
>>> get more memory soon.
>>
>> No we don't. If we keep polling we are quite possibly keeping the CPU
>> busy so delaying the hint request processing. Again the issue it's a
>
> You can always yield. But that's a different topic.
>
>> tradeoff. One performance for the other. Very hard to know which path do
>> you hit in advance, and in the real world no one has the time to profile
>> and tune things. By comparison trading memory for performance is well
>> understood.
>>
>>
>>> "appended to guest memory", "global list of memory", malicious guests
>>> always using that memory like what about NUMA?
>>
>> This can be up to the guest. A good approach would be to take
>> a chunk out of each node and add to the hints buffer.
>
> This might lead to you not using the buffer efficiently. But also,
> different topic.
>
>>
>>> What about different page
>>> granularity?
>>
>> Seems like an orthogonal issue to me.
>
> It is similar, yes. But if you support multiple granularities (e.g.
> MAX_ORDER - 1, MAX_ORDER - 2 ...) you might have to implement some sort
> of buddy for the buffer. This is different than just a list for each node.

Oh, and before I forget, different zones might of course also be a problem.

--

Thanks,

David / dhildenb