VM Host CPU resources

[Tim Nelson]

I am doing some performance testing with some VM Guests and ran into something that I did not expect.

Is the full CPU resource ( or at least most of it ) of the VM host available to the VM Guest.

I have a two CPU rx2620 with two single vCPU guests configured.

While monitoring the VM Host, if I run up the CPU usage on one of the guests with a couple do : loops, the CPU usage on the Host never goes over 50% ( or 1 CPU ).

Certainly if I assign two vCPUs to the guest I can drive both CPUs to 100%.

entitlements for VMGuest is at 10% default.

I have read some doc that eludes that the full horsepower of the VMHost is available to each guest ( barring competition from the guests ).

Any comments ?

[Jerome Forissier]

Sounds normal to me. If the guest has only one virtual CPU, you can't drive both physical CPUs to 100% because a vCPU can only be bound to one physical CPU at any given time. Basically the VM schedules vCPUs on physical CPUs, in a similar way an OS schedules threads on available CPUs.

(Well I'm not a VM expert, I'm just giving my understanding :))

[spex]

Hi Tim,

According to the VM Administration guide:

"Virtual machine entitlement is the minimum amount of processing power guaranteed to the virtual machine from each virtual CPU."

VM Host will ensure that the guest receives at least its entitlement, but is not restricted from offering more of a resource so long as doing so does not interfere with the entitlement of other guests.

http://docs.hp.com/en/T2767-90024/ch03s01.html#entitlement_section

PCS

[Torsten.]

What happens, if you adjust the entitlement to >50% for a single vCPU?

[Tim Nelson]

Jerome, That is what it seems to me, some of the virtual machine doc leads me to believe that the entire resources are available ( I did see the VM switch back and forth between VHost CPUs but again 75% on one then only 25% on the other.

Spex, The quote mentioned is one I have seen that promises a minimum but offers the maximum if available ( again not able to experience that )

Torsten, I will see if it makes a diff, my understanding is the entitlement is only the promised minimum ( will let you know ).

Another option, for example ( although the best practice states less is better ). If I have an 8 way system and present each vGuest with 8 vCPUs, then I can drive all CPUS with one guest but, the entitlement will then reserver a % of each of the 8 CPUs and reduce the potential number of vGuests.

I guess another issue may be for instance using Oracle, Oracle will tune itself to the number of CPUs it sees. In this case more is better for parallelism..

[Torsten.]

As far as I know the maximum is 4 vCPUs per VM.

My understanding is this (it is not really clear to me from the docs):

with your entitlement the 2 VMs are sharing the same physical CPU, this is the reason for the 50% on the host (the second is idle).

I guess, with an entitlement >50% each VM get 1 physical CPU (with 1 vCPU per VM).

Please test this, I would like to know this too - I haven't any test system :-(

[Tim Nelson]

Entitlement at 50% ( any higher entlmnt shows same )

14:34:55 cpu %usr %sys %wio %idle
14:34:59 0 0 100 0 0
1 0 0 0 100 system 0 50 0 50