2 times more VMs withOUT KVM

2 times more VMs withOUT KVM

[Pavel Shirshov]

Hello,

Today I was very surprised to see that KVM with enabled HugePages uses
much more memory versus KVM without HugePages enabled.

I have a server with 386Gb memory.
I have a VM image which allocate up to 3.2Gb memory (libvirt <memory/> tag).

Without HugePages enabled on the server I can create up to 235 VM on the server.

With HugePages enabled I can create about 105 VMs on the same server.
I reserve 180000 2Mb pages on the server.

I see that 180000*2Mb / 3.2 Gb = 112.5 VMs.

How KVM without HugePages is able to create 235 VMs? Does KSM help?

Is it possible to tune KVM with HugePages up to support ~200 VMs?

Thanks
-- 
With best regards, Pavel S. Shirshov

Re: 2 times more VMs withOUT KVM

[Paolo Bonzini]

On 31/07/2015 01:08, Pavel Shirshov wrote:
> Hello,
> 
> Today I was very surprised to see that KVM with enabled HugePages uses
> much more memory versus KVM without HugePages enabled.
> 
> I have a server with 386Gb memory.
> I have a VM image which allocate up to 3.2Gb memory (libvirt <memory/> tag).
> 
> Without HugePages enabled on the server I can create up to 235 VM on the server.
> 
> With HugePages enabled I can create about 105 VMs on the same server.
> I reserve 180000 2Mb pages on the server.
> 
> I see that 180000*2Mb / 3.2 Gb = 112.5 VMs.
> 
> How KVM without HugePages is able to create 235 VMs? Does KSM help?

KVM without hugepages can overcommit.  It can do this thanks to unused
memory (never touched by the VM, thus all zero), swapping out,
deduplication (KSM), ballooning or a combination of the four.

Hugepages are never overcommitted.

Paolo

> Is it possible to tune KVM with HugePages up to support ~200 VMs?
> 
> Thanks

Re: 2 times more VMs withOUT KVM

[Pavel Shirshov]

Hi Paolo,

Thank you for the answer. I suspect that overcommit thing. In my case
it's a combination of unused memory and KSM.

So hugepages are not so useful as I saw in the presentations. I think
it works just for some usecases.

Thanks

On Fri, Jul 31, 2015 at 12:54 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> On 31/07/2015 01:08, Pavel Shirshov wrote:
>> Hello,
>>
>> Today I was very surprised to see that KVM with enabled HugePages uses
>> much more memory versus KVM without HugePages enabled.
>>
>> I have a server with 386Gb memory.
>> I have a VM image which allocate up to 3.2Gb memory (libvirt <memory/> tag).
>>
>> Without HugePages enabled on the server I can create up to 235 VM on the server.
>>
>> With HugePages enabled I can create about 105 VMs on the same server.
>> I reserve 180000 2Mb pages on the server.
>>
>> I see that 180000*2Mb / 3.2 Gb = 112.5 VMs.
>>
>> How KVM without HugePages is able to create 235 VMs? Does KSM help?
>
> KVM without hugepages can overcommit.  It can do this thanks to unused
> memory (never touched by the VM, thus all zero), swapping out,
> deduplication (KSM), ballooning or a combination of the four.
>
> Hugepages are never overcommitted.
>
> Paolo
>
>> Is it possible to tune KVM with HugePages up to support ~200 VMs?
>>
>> Thanks



-- 
With best regards, Pavel S. Shirshov