Adding CPU's Question

[MikeGIllinois]

Hi.

  I've got a Windows Server 2008 Terminal Server, a Windows Server 2008,  and another Server 2008 all running as VM's on a quad core HP server.  The Terminal Server needs more "juice."

   Can I give two or even all three of the virtuals 4 CPU's in Edit Settings, CPU's? It seems counter-intuitive to me to overallocate CPU's but VMWare doesn't stop me.

     What happens if two or three virtual servers both have access to 4 CPU's? Obviously they both can't access all four cores simultaneously, can they?

     Thank you, as always, for your excellent help. <-:

Mike Gallery

[weinstein5 · Accepted solution]

It is really going to depend on how many physicl cores the ESXi host has and how many VMs are running on it  - becaue I would not place a VM with 4 vCPUs onto a host that only has 4 cores available to it -

[MikeGIllinois]

Thanks for your reply, David.

This server has four cores. The Terminal Server is used mostly. The other server is for file sharing only for a specific app. The other server is an anti-virus server.

"because I would not place a VM with 4 vCPUs onto a host that only has 4 cores available to it"

Why wouldn't you do what you describe above? What might happen?

Thank you.

Mike Gallery

[weinstein5]

It has to do with how the vCPUs get scheduled to the physical cores -  What is the current configuration of the VMs you have running on this host? Are these the only three VMs you have running on this host? My gut feeling is if you were increase the number of vCPUs performance will gte worst because you might already situations where you VMs are contending for CPU resources -

You can oversommit CPU but you want to make the CPU load is such that all the VMs are not demanding CPU at the same time -

[UmeshAhuja]

Hi,

Assuming that your esxi host has 4 Quad core CPU i.e 16 Logical cores , Now if you are giving 4 CPU to say vm A and another vm B is having 2 CPU , So in that case vm B will get faster response from vmkernel as there are less CPU in quie compare to 4 CPU vm.

i.e the CPU waits for the response till the time vmkernel have required CPU free .

Hence providing more or equivilant CPU as of esxi host will give you high CPU usage and low performance.

Hence distribute the CPU as per the server and internal application requirement depending on the physical hardware available to esxi host

[MikeGIllinois]

I currently have a 2.13 GHz Xeon QuadCore processor running on this HP DL360 Server. Performance overall is very good.

I have 2 virtual cores assigned to each of the 3 virtual machines. No either VM's are running on this setup.

I have checked and the Terminal Server users are satisified so I think I'll leave well enough alone. It's important that five or six simultaneous users not get bogged down while running various software!

Thank you for helping me clarify this.

Mike Gallery

has to do with how the vCPUs get scheduled to the physical cores -   What is the current configuration of the VMs you have running on this  host? Are these the only three VMs you have running on this host? My gut  feeling is if you were increase the number of vCPUs performance will  gte worst because you might already situations where you VMs are  contending for CPU resources -

You can oversommit CPU but you want to make the CPU load is such that all the VMs are not demanding CPU at the same time -

[MikeGIllinois]

Thank you, Umesh. The single CPU on this HP Server is a Quadcore so I only have the one processor to work with. But I will keep your advice in mind if I get into servers with multiple processors.

Mike Gallery

[weinstein5]

Yes definitely do not fix it if it is not broken - you can run into issues - 

[arminmacx]

in my idea you killing the ESXi performance and you VM by adding extra core to the VM you may get in a big issue if you do that.

its better in your situation to lower the other VM you said don want a lot vCPU and add it to you Terminal Server VM.

Best Regards

--Armin--