Re: [RFC][Patch v8 6/7] KVM: Enables the kernel to isolate and report free pages

From: Nitesh Narayan Lal
Date: Tue Feb 12 2019 - 12:10:58 EST

On 2/9/19 7:38 PM, Michael S. Tsirkin wrote:
> On Fri, Feb 08, 2019 at 02:05:09PM -0800, Alexander Duyck wrote:
>> On Fri, Feb 8, 2019 at 1:38 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>>> On Fri, Feb 08, 2019 at 03:41:55PM -0500, Nitesh Narayan Lal wrote:
>>>>>> I am also planning to try Michael's suggestion of using MAX_ORDER - 1.
>>>>>> However I am still thinking about a workload which I can use to test its
>>>>>> effectiveness.
>>>>> You might want to look at doing something like min(MAX_ORDER - 1,
>>>>> HUGETLB_PAGE_ORDER). I know for x86 a 2MB page is the upper limit for
>>>>> THP which is the most likely to be used page size with the guest.
>>>> Sure, thanks for the suggestion.
>>> Given current hinting in balloon is MAX_ORDER I'd say
>>> share code. If you feel a need to adjust down the road,
>>> adjust both of them with actual testing showing gains.
>> Actually I'm left kind of wondering why we are even going through
>> virtio-balloon for this?
> Just look at what does it do.
> It improves memory overcommit if guests are cooperative, and it does
> this by giving the hypervisor addresses of pages which it can discard.
> It's just *exactly* like the balloon with all the same limitations.
>> It seems like this would make much more sense
>> as core functionality of KVM itself for the specific architectures
>> rather than some side thing.
> Well same as balloon: whether it's useful to you at all
> would very much depend on your workloads.
> This kind of cooperative functionality is good for co-located
> single-tenant VMs. That's pretty niche. The core things in KVM
> generally don't trust guests.
>> In addition this could end up being
>> redundant when you start getting into either the s390 or PowerPC
>> architectures as they already have means of providing unused page
>> hints.
> Interesting. Is there host support in kvm?
>> I have a set of patches I proposed that add similar functionality via
>> a KVM hypercall for x86 instead of doing it as a part of a Virtio
>> device[1]. I'm suspecting the overhead of doing things this way is
>> much less then having to make multiple madvise system calls from QEMU
>> back into the kernel.
> Well whether it's a virtio device is orthogonal to whether it's an
> madvise call, right? You can build vhost-pagehint and that can
> handle requests in a VQ within balloon and do it
> within host kernel directly.
> virtio rings let you pass multiple pages so it's really hard to
> say which will win outright - maybe it's more important
> to coalesce exits.
> Nitesh, how about trying same tests and reporting performance?
Noted, I can give it a try before my next positing.
>> One other concern that has been pointed out with my patchset that
>> would likely need to be addressed here as well is what do we do about
>> other hypervisors that decide to implement page hinting. We probably
>> should look at making this KVM/QEMU specific code run through the
>> paravirtual infrastructure instead of trying into the x86 arch code
>> directly.
>> [1]
> So virtio is a paravirtual interface, that's an argument for
> using it then.
> In any case pls copy the Cc'd crowd on future version of your patches.

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