linux-kernel - Re: [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting

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Date:   Wed, 6 Mar 2019 19:59:57 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, pbonzini@...hat.com, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com, yang.zhang.wz@...il.com,
        riel@...riel.com, dodgen@...gle.com, konrad.wilk@...cle.com,
        dhildenb@...hat.com, aarcange@...hat.com, alexander.duyck@...il.com
Subject: Re: [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting

On 06.03.19 19:43, Michael S. Tsirkin wrote:
> On Wed, Mar 06, 2019 at 01:30:14PM -0500, Nitesh Narayan Lal wrote:
>>>> Here are the results:
>>>>
>>>> Procedure: 3 Guests of size 5GB is launched on a single NUMA node with
>>>> total memory of 15GB and no swap. In each of the guest, memhog is run
>>>> with 5GB. Post-execution of memhog, Host memory usage is monitored by
>>>> using Free command.
>>>>
>>>> Without Hinting:
>>>>                  Time of execution    Host used memory
>>>> Guest 1:        45 seconds            5.4 GB
>>>> Guest 2:        45 seconds            10 GB
>>>> Guest 3:        1  minute               15 GB
>>>>
>>>> With Hinting:
>>>>                 Time of execution     Host used memory
>>>> Guest 1:        49 seconds            2.4 GB
>>>> Guest 2:        40 seconds            4.3 GB
>>>> Guest 3:        50 seconds            6.3 GB
>>> OK so no improvement.
>> If we are looking in terms of memory we are getting back from the guest,
>> then there is an improvement. However, if we are looking at the
>> improvement in terms of time of execution of memhog then yes there is none.
> 
> Yes but the way I see it you can't overcommit this unused memory
> since guests can start using it at any time.  You timed it carefully
> such that this does not happen, but what will cause this timing on real
> guests?

Whenever you overcommit you will need backup swap. There is no way
around it. It just makes the probability of you having to go to disk
less likely.

If you assume that all of your guests will be using all of their memory
all the time, you don't have to think about overcommiting memory in the
first place. But this is not what we usually have.

> 
> So the real reason to want this is to avoid need for writeback on free
> pages.
> 
> Right?

-- 

Thanks,

David / dhildenb