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SubjectRe: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting
On Tue, Feb 19, 2019 at 01:57:14PM -0800, Alexander Duyck wrote:
> On Tue, Feb 19, 2019 at 10:32 AM David Hildenbrand <> wrote:
> >
> > >>> This essentially just ends up being another trade-off of CPU versus
> > >>> memory though. Assuming we aren't using THP we are going to take a
> > >>> penalty in terms of performance but could then free individual pages
> > >>> less than HUGETLB_PAGE_ORDER, but the CPU utilization is going to be
> > >>> much higher in general even without the hinting. I figure for x86 we
> > >>> probably don't have too many options since if I am not mistaken
> > >>> MAX_ORDER is just one or two more than HUGETLB_PAGE_ORDER.
> > >>
> > >> THP is an implementation detail in the hypervisor. Yes, it is the common
> > >> case on x86. But it is e.g. not available on s390x yet. And we also want
> > >> this mechanism to work on s390x (e.g. for nested virtualization setups
> > >> as discussed).
> > >>
> > >> If we e.g. report any granularity after merging was done in the buddy,
> > >> we could end up reporting everything from page size up to MAX_SIZE - 1,
> > >> the hypervisor could ignore hints below a certain magic number, if it
> > >> makes its life easier.
> > >
> > > For each architecture we can do a separate implementation of what to
> > > hint on. We already do that for bare metal so why would we have guests
> > > do the same type of hinting in the virtualization case when there are
> > > fundamental differences in page size and features in each
> > > architecture?
> > >
> > > This is another reason why I think the hypercall approach is a better
> > > idea since each architecture is likely going to want to handle things
> > > differently and it would be a pain to try and sort that all out in a
> > > virtio driver.
> >
> > I can't follow. We are talking about something as simple as a minimum
> > page granularity here that can easily be configured. Nothing that
> > screams for different implementations. But I get your point, we could
> > tune for different architectures.
> I was thinking about the guest side of things. Basically if we need to
> define different page orders for different architectures then we start
> needing to do architecture specific includes. Then if we throw in
> stuff like the fact that the first level of KVM can make use of the
> host style hints then that is another thing that will be a difference
> int he different architectures.

Sorry didn't catch this one. What are host style hints?

> I'm just worried this stuff is going
> to start adding up to a bunch of "#ifdef" cruft if we are trying to do
> this as a virtio driver.

I agree we want to avoid that.

And by comparison, if it's up to host or if it's tied to logic within
guest (such as MAX_PAGE_ORDER as suggested by Linus) as opposed to CPU
architecture, then virtio is easier as you can re-use config space and
feature bits to negotiate host/guest capabilities. Doing hypercalls for
that would add lots of hypercalls.

I CC'd Wei Wang who implemented host-driven hints in the balloon right
now. Wei I wonder - could you try changing from MAX_PAGE_ORDER to
HUGETLB_PAGE_ORDER? Does this affect performance for you at all? Thanks!

> > >
> > >>>
> > >>> As far as fragmentation my thought is that we may want to look into
> > >>> adding support to the guest for prioritizing defragmentation on pages
> > >>> lower than THP size. Then that way we could maintain the higher
> > >>> overall performance with or without the hinting since shuffling lower
> > >>> order pages around between guests would start to get expensive pretty
> > >>> quick.
> > >>
> > >> My take would be, design an interface/mechanism that allows any kind of
> > >> granularity. You can than balance between cpu overead and space shifting.
> > >
> > > The problem with using "any kind of granularity" is that in the case
> > > of memory we are already having problems with 4K pages being deemed
> > > too small of a granularity to be useful for anything and making
> > > operations too expensive.
> >
> > No, sorry, s390x does it. And via batch reporting it could work. Not
> > saying we should do page granularity, but "to be useful for anything" is
> > just wrong.
> Yeah, I was engaging in a bit of hyperbole. I have had a headache this
> morning so I am a bit cranky.
> So I am assuming the batching is the reason why you also have a
> arch_alloc_page then for the s390 so that you can abort the hint if a
> page is reallocated before the hint is processed then? I just want to
> confirm so that my understanding of this is correct.
> If that is the case I would be much happier with an asynchronous page
> hint setup as this doesn't deprive the guest of memory while waiting
> on the hint. The current logic in the patches from Nitesh has the
> pages unavailable to the guest while waiting on the hint and that has
> me somewhat concerned as it is going to hurt cache locality as it will
> guarantee that we cannot reuse the same page if we are doing a cycle
> of alloc and free for the same page size.
> > >
> > > I'm open to using other page orders for other architectures. Nothing
> > > says we have to stick with THP sized pages for all architectures. I
> > > have just been focused on x86 and this seems like the best fit for the
> > > balance between CPU and freeing of memory for now on that
> > > architecture.
> > >
> > >> I feel like repeating myself, but on s390x hinting is done on page
> > >> granularity, and I have never heard somebody say "how can I turn it off,
> > >> this is slowing down my system too much.". All we know is that one
> > >> hypercall per free is most probably not acceptable. We really have to
> > >> play with the numbers.
> > >
> > > My thought was we could look at doing different implementations for
> > > other architectures such as s390 and powerPC. Odds are the
> > > implementations would be similar but have slight differences where
> > > appropriate such as what order we should start hinting on, or if we
> > > bypass the hypercall/virtio-balloon for a host native approach if
> > > available.
> > >
> > >> I tend to like an asynchronous reporting approach as discussed in this
> > >> thread, we would have to see if Nitesh could get it implemented.
> > >
> > > I agree it would be great if it could work. However I have concerns
> > > given that work on this patch set dates back to 2017, major issues
> > > such as working around device assignment have yet to be addressed, and
> > > it seems like most of the effort is being focused on things that in my
> > > opinion are being over-engineered for little to no benefit.
> >
> > I can understand that you are trying to push your solution. I would do
> > the same. Again, I don't like a pure synchronous approach that works on
> > one-element-at-a-time. Period. Other people might have other opinions.
> > This is mine - luckily I don't have anything to say here :)
> >
> > MST also voted for an asynchronous solution if we can make it work.
> > Nitesh made significant improvements since the 2017. Complicated stuff
> > needs time. No need to rush. People have been talking about free page
> > hinting since 2006. I talked to various people that experimented with
> > bitmap based solutions two years ago.
> Now that I think I have a better understanding of how the s390x is
> handling this I'm beginning to come around to the idea of an
> asynchronous setup. The one thing that has been bugging me about the
> asynchronous approach is the fact that the pages are not available to
> the guest while waiting on the hint to be completed. If we can do
> something like an arch_alloc_page and that would abort the hint and
> allow us to keep the page available while waiting on the hint that
> would be my preferred way of handling this.
> > So much to that, if you think your solution is the way to go, please
> > follow up on it. Nitesh seems to have decided to look into the
> > asynchronous approach you also called "great if it could work". As long
> > as we don't run into elementary blockers there, to me it all looks like
> > we are making progress, which is good. If we find out asynchronous
> > doesn't work, synchronous is the only alternative.
> I plan to follow up in the next week or so.
> > And just so you don't get me wrong: Thanks for looking and working on
> > this. And thanks for sharing your opinions and insights! However making
> > a decision about going your way at this point does not seem reasonable
> > to me. We have plenty of time.
> I appreciate the feedback. Sorry if I seemed a bit short. As I
> mentioned I've had a headache most of the morning which hasn't really
> helped my mood.
> Thanks.
> - Alex


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