Re: On guest free page hinting and OOM

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Mar 29, 2019 at 04:45:58PM +0100, David Hildenbrand wrote:
> 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.

Right but we don't plan to do it yet.

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

I would just split the hint buffer evenly between zones.

> 
> -- 
> 
> Thanks,
> 
> David / dhildenb