Re: 2 times more VMs withOUT KVM

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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@xxxxxxxxxx> 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
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