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

[David Hildenbrand]

On 06.03.19 20:08, Alexander Duyck wrote:
> On Wed, Mar 6, 2019 at 11:00 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> 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.
> 
> Right, but the general idea is that free page hinting allows us to
> avoid having to use the swap if we are hinting the pages as unused.
> The general assumption we are working with is that some percentage of
> the VMs are unused most of the time so you can share those resources
> between multiple VMs and have them free those up normally.

Yes, similar to VCPU yielding or playin scheduling when the VCPU is
spleeping. Instead of busy looping, hand over the resource to somebody
who can actually make use of it.

> 
> If we can reduce swap usage we can improve overall performance and
> that was what I was pointing out with my test. I had also done
> something similar to what Nitesh was doing with his original test
> where I had launched 8 VMs with 8GB of memory per VM on a system with
> 32G of RAM and only 4G of swap. In that setup I could keep a couple
> VMs busy at a time without issues, and obviously without the patch I
> just started to OOM qemu instances and  could only have 4 VMs at a
> time running at maximum.

While these are nice experiments (especially to showcase reduced swap
usage!), I would not suggest to use 4GB of swap on a x2 overcomited
system (32GB overcommited). Disks are so cheap nowadays that one does
not have to play with fire.

But yes, reducing swap usage implies overall system performance (unless
the hinting is terribly slow :) ). Reducing swap usage, not swap space :)

-- 

Thanks,

David / dhildenb