White Paper - Intel Intelligent Power Node Manager

4. PMBus and Power Supply sensors to enable NM support
The Power Management Bus (“PMBus”) is an open standard protocol that defines a
means of communicating with power conversion and other devices.
For more information, please see the System Management Interface Forum Web site:
www.powerSIG.org
.
To support NM capabilities the server should have the power supply device and power supply unit
compliant with PMBus specification.
5. Node Manager FAQs for finer control of the server system
1. I have created and enabled the policy. Why is it not effective?
There could be multiple reasons for having observed this behavior. Once a policy has been set, it
should be effective and should limit the power consumption of the system within the set limit. But for
the policy to be really effective following things are necessary –
a. Node Manager Policy should be enabled
b. PM Bus power supplies with updated PDB firmware (see the tables for finding compatible
power supplies for given server in the table above)
c. The system should be sufficiently loaded to observe the effective/optimum behavior ( cpu
usage should be > 25% approximately)
d. The power limit set should be greater than the idle power consumption of the system.
Even if all these conditions are met, sometimes you may not see the power consumption of the
system coming down to the set power limit of policy. This happens because of two things
e. The system is working at a very high load and the power limit set is very low, so even by
using the least P State, Node Manager is not able to bring the power value to the set power
limit.
f. T states in the system are not enabled due to which NM is not able to make use of T states
and bring the power further down.
2. How do you enable T States?
The example here is on Windows 2008 Enterprise edition. T States work after enabling special power
policy (quoting from "Processor Power Management in Windows Vista and Windows Server Longhorn"
by Microsoft):
a). Open an elevated command prompt.
b). View the “Allow Throttle States processor power policy” by using the following command:
powercfg -qh scheme_current sub_processor
Power Scheme GUID: 381b4222-f694-41f0-9685-ff5bb260df2e (Balanced)
Subgroup GUID: 54533251-82be-4824-96c1-47b60b740d00 (Processor power
management)
Power Setting GUID: 3b04d4fd-1cc7-4f23-ab1c-d1337819c4bb (Allow Throttle
States)
Possible Setting Index: 000