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Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting
To:     "Wang, Wei W" <wei.w.wang@intel.com>,
        'Nitesh Narayan Lal' <nitesh@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
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        "lcapitulino@redhat.com" <lcapitulino@redhat.com>,
        "pagupta@redhat.com" <pagupta@redhat.com>,
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        "konrad.wilk@oracle.com" <konrad.wilk@oracle.com>,
        "dhildenb@redhat.com" <dhildenb@redhat.com>,
        "aarcange@redhat.com" <aarcange@redhat.com>
References: <20190204201854.2328-1-nitesh@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <401883fd-bdad-6e34-cabe-c9b8fdadce92@redhat.com>
Date:   Thu, 14 Feb 2019 10:34:28 +0100
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On 14.02.19 10:12, Wang, Wei W wrote:
> On Thursday, February 14, 2019 1:22 AM, Nitesh Narayan Lal wrote:
>> In normal condition yes we would not like to report any memory when the
>> guest is already under memory pressure.
>>
>> I am not sure about the scenario where both guest and the host are under
>> memory pressure, who will be given priority? Is it something per-decided or
>> it depends on the use case?
>>
> 
> That's one of the reasons that I would vote for "host to ask for free pages".

As I already said, there are scenarios where this does not work reliably.

When we

1. Let the guest report free pages
2. Allow the hypervisor to enable/disable it

We have a mechanism that is superior to what you describe.

"host to ask for free pages" can be emulated using that.

> 
> Host can have a global view of all the guest's memory states, so better to have the
> memory overcommitment policy defined on the host.
> 
> For example, the host can know guest1 is under memory pressure (thus not asking him)
> and guest2 has a huge amount of free memory. When host lacks memory to run the 3rd
> guest, it can asks guest2 to offer some free memory. 
> 
> Best,
> Wei
> 


-- 

Thanks,

David / dhildenb