linux-kernel - Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

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Date:   Thu, 14 Feb 2019 10:34:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Wang, Wei W" <wei.w.wang@...el.com>,
        'Nitesh Narayan Lal' <nitesh@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lcapitulino@...hat.com" <lcapitulino@...hat.com>,
        "pagupta@...hat.com" <pagupta@...hat.com>,
        "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "riel@...riel.com" <riel@...riel.com>,
        "dodgen@...gle.com" <dodgen@...gle.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "dhildenb@...hat.com" <dhildenb@...hat.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>
Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

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