RHEVM : RHEV Reports are not reflective of VM based performance stats

Hi

Can anyone explain why the output from RHEV Reports and CPU Usage in particular, is not reflective of those being recorded on the VM itself via sar.

For instance, looking at a RHEV Report of CPU Usage for a VM over a single 24hr period, its telling me that it was comsuming around 75% of its allocated vCPU.

Compared to the results from sar on the VM itself which tells me it was consuming between 30 and 40% over the same 24hr period.

I had a similar query some time ago regarding the memory usage being displayed via the webadmin tool against analysis on the VM itself and the discrepancies between the two.  The answer that came back was that the amount reported as being consumed by the hypervisor and by the VM itself , would always be different.   I never really understood why.

In the case of RHEV Reports CPU Usage and sar, why are they SO wildly different ?

I have a supprt case open, natch.

Cheers