HP Integrity NonStop NB54000c Reference manual

HP Power Capping and HP Dynamic Power Capping for
ProLiant servers
Technology brief, 2nd Edition
Introduction......................................................................................................................................... 3
Basics of server power control............................................................................................................... 3
Processor P-states ............................................................................................................................. 4
Clock throttling................................................................................................................................. 4
How power capping functions............................................................................................................... 5
Maintaining power consumption below the cap................................................................................... 5
Minimum and maximum power consumption for a server...................................................................... 5
Differences between HP Dynamic Power Capping and HP Power Capping................................................. 6
Power provisioning and Dynamic Power Capping................................................................................ 6
Support for Power Capping in ProLiant servers .................................................................................... 7
Enclosure Dynamic Power Capping ....................................................................................................... 8
Elements of an enclosure power cap................................................................................................... 8
Operation of Enclosure Dynamic Power Capping................................................................................. 9
Active power reallocation ............................................................................................................... 10
Enclosure Dynamic Power Capping in mixed blade environments ........................................................ 10
Opting out servers.......................................................................................................................... 10
Setting power caps for servers............................................................................................................. 10
Setting a power cap for a single server............................................................................................. 11
Setting a power cap for a group of servers ....................................................................................... 13
Setting a BladeSystem enclosure power cap...................................................................................... 13
Setting a power cap for a group of enclosures................................................................................... 14
Using power capping in data center provisioning.................................................................................. 15
Choosing effective power caps ........................................................................................................ 15
Power capping to peak power consumption ...................................................................................... 17
Power capping to average power consumption ................................................................................. 19
Using Enclosure Dynamic Power Capping in power provisioning............................................................. 19
Power capping for emergency management...................................................................................... 20
Time-of-day power capping ............................................................................................................. 21
Subtleties of power capping................................................................................................................ 22
Avoiding power capping conflicts within groups................................................................................ 22
Powering up groups of servers when using Dynamic Power Capping ................................................... 22
Setting low or unattainable power caps on servers............................................................................. 22
Peak power reporting and Dynamic Power Capping .......................................................................... 23
Using HP Power Regulator in conjunction with power capping ............................................................ 23
Power capping and CPU utilization .................................................................................................. 23
Power capping and option cards ..................................................................................................... 23

Providing a guardband for a power capping group ........................................................................... 23
Summary .......................................................................................................................................... 24
For more information.......................................................................................................................... 25
Call to action .................................................................................................................................... 25

3
Introduction
Server performance-per-watt continues to increase steadily. However, the number of watts-per-server also continues
to climb steadily. These increases, combined with the increasing number of servers and density in modern data
centers, make planning and managing facility power and cooling resources critically important. HP Power Capping
and HP Dynamic Power Capping are ProLiant power management tools that assist the data center administrator in
these critical tasks.
HP implements both Power Capping and HP Dynamic Power Capping in system hardware and firmware. Therefore,
they are not dependent on the operating system or applications. Power capping uses the power monitoring and
control mechanisms built into ProLiant servers. These mechanisms allow an administrator to limit, or cap, the power
consumption of a server or group of servers. Power capping lets you manage the data center parameters that server
power consumption directly influences, including data center cooling requirements and electrical provisioning.
Power capping also lets you control server power consumption in emergencies such as loss of primary AC power.
It is important to understand that power capping does not reduce the total energy consumption required for a server
to accomplish a given computational workload. Power capping simply limits the amount of power that a server can
use at any point in time. This lets you allocate data center power and cooling resources more efficiently. In general,
if a given power cap restricts the amount of power that a server would normally use to perform a task, that task will
take longer. Over time, the server will consume about the same total energy to execute the same computational
workload.
This paper outlines the use of power capping as part of a planning and provisioning strategy in the data center. It
also describes the relationship between power capping and other power management tools such as HP Power
Regulator.
Basics of server power control
The processor complex is one of the single greatest power consumers in ProLiant servers. In many common
configurations it is responsible for one-third of the power a server consumes (Figure 1). It also indirectly drives the
power consumption of other server components. A busy processor naturally increases the workload in both the
memory and peripherals. The heat generated by the increased workload causes the fans to work harder.
All HP power management technologies use this processor-driven model to control the processor’s power
consumption directly and to control overall server power consumption indirectly. The power management system
accomplishes this control using two separate mechanisms: changing the processor P-state and throttling the
processor clock.

4
Figure 1. Power use in a typical server
Processor P-states
Processor performance states, or P-states, provide a quick and effective mechanism for adjusting processor power
consumption and performance. Both Intel®and AMD®processors support using P-states to decrease processor
power consumption by lowering the processor’s core frequency and voltage. Tables 1 and 2 list some of the P-states
available with different processors.
Table 1. P-states of the Intel Xeon 5160 processor
P-state Core Frequency
P0 3.0 GHz
P1 2.66 GHz
P2 2.33 GHz
P3 2.0 GHz
Table 2. P-states of the AMD Opteron 2220 processor
P- state Core Frequency
P0 2.8 GHz
P1 2.6 GHz
P2 2.4 GHz
P3 2.2 GHz
P4 2.0 GHz
P5 1.8 GHz
P6 1.0 GHz
Clock throttling
Clock throttling is another method for lowering processor power consumption. Depending on the processor model,
the system BIOS can either reprogram the processor to run at a lower frequency or modulate the processor between
running periods and stopped periods. Both methods have the same net effect of lowering the processor’s overall
power consumption below the levels available using P-states. The chart in Figure 2 illustrates the relationship
between consumed power and overall performance when using P-states and clock throttling to control server power.
Using P-states clearly provides greater power reduction for a smaller loss in performance. However, using P-states
can lower power consumption only to a certain point. Reducing consumption below that point requires the use of
clock throttling.
Typical Server Power Usage
Processor
33%
Fans
7%
Memory
28%
PCI Slots
15%
Hard Drives
4%
Base Load
13%

5
Figure 2. Power versus performance characteristics for a typical Intel-based ProLiant server with three P-states
How power capping functions
Maintaining power consumption below the cap
With power capping, an administrator can set a maximum power consumption level for an individual server or for
a group of servers. The ProLiant power management system constantly monitors server power use. It adjusts P-states
and/or clock throttling to limit processor power use and control overall system power consumption. As needed, the
power capping control mechanism lowers the server’s power consumption in a controlled manner to keep it below
the cap―without affecting the server workload or environment.
Server power consumption depends on many factors and can vary significantly over a given period. Some factors,
such as the number of options installed in the server, have a predictable and static effect on server power
consumption. Other factors have a dynamic effect on power consumption, for example the temperature in the data
center; the activity of the CPU, memory, disk drives, and I/O subsystems; and even the mix of instructions executed.
As long as the power consumption remains below the power cap, the power capping control mechanism takes no
action and there is no affect on server performance.
Minimum and maximum power consumption for a server
The power management system in each server determines both the minimum and maximum power consumption for
the server. It determines these two values during the server’s power-on self test (POST) by executing tests measuring
server power consumption in idle mode and under a simulated maximum load. Because they are determined
empirically, the two power values implicitly take into account the server configuration and its current physical
environment. The Insight Control and iLO interfaces display both values to provide key information that
administrators can use to set effective power caps.
The power management system in the server supplies one additional metric: maximum available power for the
server. For ProLiant ML and ProLiant DL servers, this value is the maximum power that the server power supply can
DL360 G4 Power vs. Work Done
0
50
100
150
200
250
300
350
400
0500 1000 1500 2000 2500 3000 3500 4000 4500
Iterations
Watts
P0
P1
P2
Performance
Watts
P-state slope
Clock Throttling slope
1.00.25 0.750 0.5
Relative Performance

6
produce. However, the enclosure’s power supply array powers HP BladeSystem servers. For a blade server, the
maximum available power is the amount of power that the enclosure’s Onboard Administrator reserved for that
server blade. Both iLO and Insight Control report this value: iLO reports it as Power supply maximum power for ML
and DL servers and as Initial power-on request value for BladeSystem servers.
Minimum and maximum power consumption values for a server can change slightly while the server is running.
During normal operations, iLO and the power management system continue to check both the 10-second average
and the peak power readings for the server. iLO will raise the maximum power consumption level if it measures a
peak value above the established maximum. iLO will lower the minimum power consumption if it reads an average
power value that is below the present minimum.
Differences between HP Dynamic Power Capping and HP Power
Capping
Both HP Dynamic Power Capping and HP Power Capping maintain a server’s power consumption at or below the
cap value set by an administrator. HP Dynamic Power Capping monitors power consumption and maintains a
server’s power cap much faster than HP Power Capping. Table 3 provides a quick architectural and operational
comparison of HP Dynamic Power Capping and HP Power Capping. To avoid confusion between the two, we will
refer to HP Power Capping as basic Power Capping throughout the remainder of this paper.
Table 3. Characteristics of Dynamic Power Capping and basic Power Capping
Dynamic Power
Capping
Basic Power Capping
Power capping
executed from
Power management
microcontroller
iLO and system ROM
BIOS
Control of processor
power
Direct hardware
connection to
processor to control P-
state/clock throttling at
the processor
hardware level
Firmware control of P-
state/clock throttling
through processor
registers
Power monitoring cycle More than 5 times per
second
Once every 5 seconds
Time to bring server
power consumption
back under its cap
Less than 0.5 seconds 10 – 30 seconds
Intended application Managing power and
cooling provisioning
Managing cooling
provisioning
Power provisioning and Dynamic Power Capping
Basic Power Capping does an excellent job of maintaining average server power utilization at or below a cap
value. Administrators can use it to help manage data center cooling requirements: Limiting server power
consumption fast enough can prevent excessive heat generation. However, as the information in Table 3 illustrates,
basic Power Capping cannot respond quickly enough to limit sudden increases in server power consumption that
could trip an electrical circuit breaker.
Dynamic Power Capping operates more than 25 times faster than basic Power Capping. Dynamic Power Capping
can bring a server experiencing a sudden workload increase back under its power cap in less than one-half second.
This is fast enough to prevent any surge in power demand that could cause a typical data center circuit breaker to

7
trip. HP has tested Dynamic Power Capping to ensure that it can prevent tripping circuit breakers that have a
specified trip time of 3 seconds or longer at 50 degrees C and 150 percent overload.
Dynamic Power Capping can keep server power consumption below the power cap in real time. Therefore,
administrators can use it as an effective tool in planning and managing both electrical provisioning and cooling
requirements in the data center. An administrator can electrically provision a PDU or a rack to something less than
the full faceplate power rating of all the servers supported because Dynamic Power Capping will prevent a sudden
power demand from exceeding the power cap and tripping a circuit breaker.
Support for Power Capping in ProLiant servers
ProLiant servers with power measurement circuitry support basic Power Capping:
•ProLiant G5 servers ML350, ML370, DL360, DL365, DL380, and DL385
•All c-Class BladeSystem servers
Basic Power Capping requires the following system firmware:
•iLO 2 version 1.30 or later
•System BIOS 2007.05.01 or later
Support for Dynamic Power Capping requires a certain level of ProLiant hardware, as well upgrades to the
following system firmware:
•System BIOS 2008.11.01 or later
•iLO 2 version 1.70 or later
•Onboard Administrator firmware version 2.32 or later (for HP BladeSystem enclosures)
At introduction, support for Dynamic Power Capping is available on a limited set of ProLiant servers and a larger set
of ProLiant c-Class server blades. Many ProLiant G5 servers can support Dynamic Power Capping if they have fully
qualified BIOS and iLO firmware. Please consult the most recent support matrix.
Group power capping for servers through Insight Control
One of the most powerful uses of power capping is monitoring and controlling the power use of an entire group of
servers. This capability is available through Insight Control. Administrators can apply a group power cap to any
group of servers that they can select within Insight Control, including Insight collections.
Insight Control displays the aggregated minimum and maximum power consumption for an entire group of servers
and their aggregated power supply rating. Each of these numbers is simply the sum of the respective values for the
individual servers in the group. Using the Insight Control interface, an administrator can apply to the server group a
power cap that is between the minimum power and the power supply rating of the entire group.
Insight Control assigns an individual power cap to each server in the group. It is a proportional allocation of the
group power cap. The total of the individual power caps equals the group cap. The individual power caps for the
servers continue until an administrator changes them through the iLO or Insight Control interface.
Figure 3 shows a group consisting of four servers. The left side of the figure shows the measured minimum and
maximum power consumption for each server. The right side of the figure shows that the aggregated maximum
power consumption for this group is 1375 watts. The aggregated minimum allowable power consumption is 725
watts. In this example, an administrator has applied a power cap of 1115 watts to the group. This group cap limits
the group power consumption to 60 percent of the wattage between the aggregated minimum and maximum. To
implement this group power cap, Insight Control applies to each server a power cap that is 60 percent of the
wattage between that server’s minimum and maximum power consumption. This results in individual power caps of
320, 170, 305, and 320 watts respectively.

8
Figure 3. Apportioning a group power cap to individual servers in the group
Group power capping apportionment works exactly the same way on ProLiant ML and DL servers with either
Dynamic Power Capping or basic Power Capping. For server blades, there is a more advanced feature called
Enclosure Dynamic Power Capping.
Enclosure Dynamic Power Capping
Enclosure Dynamic Power Capping is a special implementation of Dynamic Power Capping for HP BladeSystem
enclosures. In one sense, it is a higher level of power management functionality, since an administrator sets and
maintains a power cap at the enclosure level rather than directly at the server or blade level. In another sense, it is a
more powerful implementation of group power capping for an enclosure: Setting a power cap for the enclosure
indirectly creates power caps for the server blades within it. The Onboard Administrator (OA) then actively
manages these power caps and reallocates power as workloads change over time.
Elements of an enclosure power cap
With Enclosure Dynamic Power Capping, an administrator sets a power cap for an entire BladeSystem enclosure,
not simply for the server blades in the enclosure. Total power consumption for an enclosure is the sum of the power
used by all of these components:
•Server blades
•I/O peripherals for the enclosure (interconnects, etc.)
•Cooling fans for the enclosure
•Onboard Administrator(s)
Group Power Capping
Individual Servers
400 watts (Maximum)
200 watts (Minimum)
Servers as a Group
320 watts (Apportioned Cap)
200
125
170
375
200
400
200
305
320
1375 watts
(Group maximum)
725 watts
(Group minimum)
1115 watts
(User set group power cap)
1000 watts (Power supply maximum)
1000
1000
500
3500 watts
(Group power supply
maximum)

9
An administrator can set the enclosure power cap to any value between the lowest total power consumption and the
maximum available power for the enclosure. Adding these values yields the lowest reasonable power cap
maintainable for the enclosure under all operating conditions:
•Total power that the server blades would use in their lowest power-capped state (typically about halfway between
server idle and server maximum power)
•Maximum power that fans in the enclosure could draw
•Power-on power requests from the other elements in the enclosure
The maximum available power for the enclosure is the total power available from the enclosure power supplies.
Operation of Enclosure Dynamic Power Capping
Once an administrator sets an enclosure power cap, the enclosure OA monitors and maintains the cap. The OA
cannot control the power consumption of unmanaged elements in the enclosure (fans, switches, and others). The OA
maintains the overall enclosure cap by monitoring and managing the power consumption of the server blades. To
accomplish this, the OA collects the overall enclosure power use and the total power used by the managed server
blades. From this data, the OA constructs a blade power budget representing the amount of power that the blades
can consume while keeping the overall enclosure power consumption below the enclosure cap. As a final step, OA
software actively adjusts the power caps of the individual servers so that the total matches the enclosure power
budget. This process repeats every 20 seconds to maintain the enclosure power cap. As Figure 4 illustrates, the OA
increases the blade power budget and the power caps for the individual server blades if power consumption of the
unmanaged elements in the enclosure decreases.
Figure 4.Power budgeting in Enclosure Dynamic Power Capping
Blade Power
Budget
Power used
by unmanaged
elements
Enclosure
Dynamic
Power Cap
Power Cap
Lower Bound
Enclosure
Dynamic
Power Cap
Time A Time B
Enclosure Dynamic Power Capping
Power Budgeting
Power Cap
Lower Bound

10
Active power reallocation
One of the more important features of Enclosure Dynamic Power Capping is the active reallocation of power
amongst the server blades over time. After each monitoring cycle, Enclosure Dynamic Power Capping actively
reapportions the individual power caps of the server blades based on their individual workloads. With the blade
power budget as its limit, the OA software uses a sophisticated, multi-tiered algorithm to increase the power caps of
individual servers that are busier and using more power. It decreases the caps for server blades using less power
from cycle to cycle. Thus, the OA optimizes the power use among the server blades in the enclosure while keeping
overall power consumption below the enclosure power cap. To maintain control of the server power caps, Enclosure
Dynamic Power Capping disables external changes to the server caps using either iLO or Insight Control.
In most cases, the OA can quickly raise a low power cap for an idle server blade that receives new work. In such
cases, the power-sharing algorithm has little impact on performance. However, if there are too many busy server
blades for the available power, the OA will attempt to share the available power fairly among all busy server
blades.
Enclosure Dynamic Power Capping in mixed blade environments
Enclosure Dynamic Power Capping operates with all server blades that support basic Power Capping or the faster
Dynamic Power Capping. It also provides circuit breaker protection using either of these types of server blades. To
accomplish this, Enclosure Dynamic Power Capping relies on the extra circuit capacity of enclosures configured with
N+N redundant power. Basic Power Capping cannot bring server blades under their caps within the 3 seconds
required for normal circuit breaker protection. However, the redundant side of the enclosure power can absorb the
transient overage until overall power consumption is again under the enclosure cap. If N+N power redundancy
fails, then a hardware-based failsafe mechanism overrides the Enclosure Dynamic Power Capping and immediately
lowers all server blade processors to a predetermined power state that prevents a circuit breaker overload. This
hardware override continues until power redundancy is restored.
Opting out servers
An administrator may want to leave some server blades uncapped, even though that may result in lower power
caps for the other server blades in the enclosure. Typically, uncapped servers run mission critical or consistently high
workload applications that require unconstrained power consumption. This allows them to maintain high throughput
and low latency response times. Enclosure Dynamic Power Capping allows an administrator to “opt out” of power
management for up to one quarter of the server blades in an enclosure, making them unmanaged elements. Looking
again at Figure 4, the OA measures and tracks the power these servers consume, but it is part of the power used by
the unmanaged elements pool. The enclosure power cap remains the same, so the blade power budget for the
remaining managed server blades becomes smaller.
Setting power caps for servers
Administrators can use iLO or Insight Control to set individual power caps for servers. Using Insight Control, you can
also set power caps for groups of ProLiant ML and DL servers and for groups of enclosures. For individual
enclosures, you can set power caps using the OA or Insight Control. You set power caps in exactly the same way
on servers supporting Dynamic Power Capping or basic Power Capping. Servers supporting Dynamic Power
Capping simply enforce the cap using the faster power management architecture.

11
Setting a power cap for a single server
Administrators can set a power cap for an individual server or server blade through iLO or through Insight Control.
In the iLO 2 browser-based interface (Figure 5), Power Capping Settings
is located beneath the Power Management
tab in the Settings subsection. The iLO 2 Power Management Settings screen indicates whether the server supports
Dynamic Power Capping.
Figure 5. Setting a power cap for an HP BladeSystem server using iLO 2

12
In Insight Control, Power Capping is located beneath the HP Power Management Actions section of the interface, as
shown in Figure 6.
Figure 6. Setting a power cap for an HP BladeSystem server using Insight Control

13
Setting a power cap for a group of servers
Insight Control is the sole tool for setting power caps for groups of ProLiant ML and DL servers. Using the Insight
Control interface (Figure 7), an administrator can apply a group power cap that is between the minimum power
and the power supply rating of the entire group.
Figure 7.Setting a group power cap in Insight Control
Setting a BladeSystem enclosure power cap
You can use either the OA or Insight Control to set the power cap for a BladeSystem enclosure to any value
between the power cap lower bound and the maximum available power for the enclosure. Figure 8 shows the
Power Management screen from HP Onboard Administrator. This screen is where you set the Enclosure Dynamic
Power Cap.

14
Figure 8. Setting an Enclosure Dynamic Power Cap in HP Onboard Administrator 2.60
Setting a power cap for a group of enclosures
An administrator can use Insight Control to apply enclosure dynamic power caps to a group of enclosures at the
same time. Figure 9 shows five BladeSystem c-Class Enclosures configured as a group in Insight Control. The
maximum available power for the group is 31,784 watts. The lower bound of the power cap is 15499 watts. It is
higher than the maximum power measurement because it represents the lowest reasonable power cap for the group
that is sustainable under all operating conditions. Applying a power cap value of 25,000 watts to the group
causes Insight Control and the OAs to create and apply a separate Enclosure Dynamic Power Cap to each of the
five enclosures, using the same proportional allocation method described earlier for applying a group power cap
to ML and DL servers. The OA within each enclosure will use this power cap to determine a blade power budget for

15
the enclosure and to create power caps for the individual server blades. It will then monitor and adjust the
individual server blade power caps based on their power demands.
Figure 9. Setting a power cap for a group of enclosures in Insight Control
Using power capping in data center provisioning
Power capping is a tool to help IT organizations manage infrastructure size and costs in the data center. Setting a
power cap for a server or group of servers assures a data center manager that unexpected changes in workload or
environment will not cause servers to consume more than the specified amount of power.
Appropriate use of basic Power Capping in the data center lets administrators provision the cooling infrastructure at
an effective level. Effective provisioning meets cooling requirements associated with server power consumption at a
realistic application load, when having all servers running at full power is unrealistic. Alternatively, using basic
Power Capping to limit average server power consumption will allow operating more servers safely within a pre-
existing cooling infrastructure.
Dynamic Power Capping takes this one step further. Its ability to control server power consumption in real time
allows administrators to use power capping for planning and managing both their electrical provisioning and their
cooling infrastructure.
Choosing effective power caps
Setting an effective power cap involves determining the lowest value for the power cap that will still meet the power
requirements for a server or group of servers running their application workload. An effective power cap should
have no affect on server application performance but would reduce the required cooling and electrical provisioning
for the servers.
Administrators can use the HP Power Calculator Utility to estimate the maximum input power for a given server
configuration. This information provides a starting point for considering power caps. Once you have deployed an
application in a laboratory or in production, reporting features in Insight Control and iLO provide historical power
consumption data. You can use that data to refine the power cap selection to achieve specific power savings or
capacity planning targets.
Figure 10 shows the output from the HP Power Calculator utility for a ProLiant DL380 G5 server configured with two
Quad-Core Intel Xeon X5460 3.16-GHz processors, one 72-GB disk drive, and 8 GB of system memory. The results

16
indicate that the Total System Input power requirement for this server configuration is 423 watts. This is the
predicted maximum amount of power that this server configuration would use under environmental and load
conditions. It is one of the numbers that an administrator could use when calculating the maximum power and
cooling requirements to support this system in the data center. For a rack containing a group of eight of these
servers, the total requirements would be 3384 watts.
Figure 10. HP Power Calculator results for a configured ProLiant DL380 G5 server

17
Continuing with this example, Figure 11 shows the Insight Control Group Power Consumption graph for the group
of eight servers running a typical variable load application. Insight Control generates this graph using data
collected from each server’s power management system. The power management system measures power
consumption twice per second and records both the peak and average power consumption. For peak power
consumption, each point in the graph represents the highest half-second power measurement recorded during a
given 5-minute period. The average power consumption denotes the arithmetic mean of all half-second power
measurements recorded over the same timeframe.
Figure 11. Group Power Consumption graph for a group of eight ProLiant DL380 G5 servers
The peak power consumption for the server group running this particular workload is about 3116 watts. The
average power consumption is about 1900 watts. Knowing the history of peak and average power consumption for
the group provides a good starting point for deciding how to set effective power caps that meet different power
management objectives in the data center.
Power capping to peak power consumption
As the power consumption graph in Figure 11 shows, peak power consumption for the eight-server group running
this workload is consistently about 3116 watts. Setting a power cap at this level would have no affect on overall
server performance. A 3116-watt power cap will ensure that the server group’s power consumption will not
normally exceed 3116 watts for any significant period. For basic Power Capping, an administrator could safely
define cooling requirements for the server group against 3116 watts of power consumed rather than against the
larger maximum power number of 3384 watts that the HP Power Calculator Utility shows for this configuration. A
guardband (described later in this paper) is not necessary in this example for two reasons. First, eight servers are
enough for the average meter error to be small. Second, in this example the caps are set to the measured peak,

18
which is high in the dynamic range of the servers. So the effects of configuration and environmental changes are
ignored. Table 4 shows a summary of the power consumption and capacity savings for the server group.
Table 4. Power consumption for eight DL380 G5 servers when capping to peak power consumption using basic Power Capping
Description
Power
Maximum possible power consumption for 8 DL380 G5 servers
(based on HP Power Calculator)
3384 watts
Maximum power consumption when capping to peak 3116 watts
Savings in power capacity 268 watts
Additional servers that can be provisioned within the same cooling infrastructure 0.7
If all eight servers supported Dynamic Power Capping, administrators could use the 3116-watt power cap to
manage electrical provisioning rather than provisioning to the faceplate value of the power supply. Table 5
illustrates how many more servers the same circuit could power under the 3116-watt power cap. Derating the
faceplate value by 20 percent requires 7760 watts. This example reserves a 10 percent guardband for other factors
(see “Providing a guardband for a power capping group”). With Dynamic Power Capping applied, the same
circuit can support over 10 more servers, if all 10 have the same power cap as the first eight servers.
Table 5. Power provisioning for eight DL380 G5 servers when capping to peak power consumption using Dynamic Power
Capping
Description
Power
Traditional input power provisioning for 8 DL380 G5 servers
(1217 input VA at 208 volts de-rated by 20 percent)
7760 watts
(970 x 8)
Maximum power consumption when capping to peak 3116 watts
Guardband 312 watts
Savings in power capacity 4332 watts
Additional servers that can be provisioned within same 7760 watts using Dynamic Power
Capping when capping to peak
10.1
It is important to understand this: Capping to peak based on the power consumption graph for a server group is
actually capping to the peak of the total power consumptions of servers in the group. It assumes that all the servers
in the group consume the same amount of power. This would not be true under these conditions:
•If the servers were configured differently
•If workloads on individual servers peaked at different times
•If workloads varied significantly from server to server
If all servers in the group did not consume the same amount of power, the cap in this example would have an
adverse affect on server performance, especially if they support Dynamic Power Capping. If the servers in a group

19
are not all the same, HP recommends using Insight Control to review the peak power consumption of each server to
ensure that the selected Dynamic Power Cap will not significantly limit its actual peak power consumption.
Power capping to average power consumption
An administrator can further increase provisioning capacity by capping the example group shown in Figure 11 at
the average power consumption: 1900 watts. Capping to average power consumption should not significantly
affect overall average computing throughput of servers running fairly uniform workloads. However, it may increase
latency during workload peaks or marginally affect the overall server performance in some applications.
The case shown in Table 6 reserves a 10 percent guardband for other factors (see “Providing a guardband for a
power capping group”). Even so, capping to average with basic Power Capping allows provisioning of over
60 percent more servers within a given cooling infrastructure.
Table 6. Power consumption for eight DL380 G5 servers when capping to average power consumption using basic Power
Capping
Description
Power
Maximum power consumption for 8 ProLiant DL580 G5 servers
(based on HP Power Calculator)
3384 watts
Maximum power consumption when capping to average 1900 watts
Guardband 190 watts
Savings in power capacity 1294 watts
Additional servers that can be provisioned within the same cooling infrastructure 5
Dynamic Power Capping will more significantly affect any workload transients exceeding the cap. Therefore, HP
recommends capping a server group supporting Dynamic Power Capping at the average power consumption only
in applications where peak transient performance is not a concern. Higher guardbands for the group might be
appropriate as well.
Using Enclosure Dynamic Power Capping in power provisioning
Dynamic Power Capping can control server power consumption quickly enough to prevent transient server power
demands that may trip circuit breakers in the data center. Using Enclosure Dynamic Power Capping, an
administrator can essentially cap the electrical provisioning for an enclosure by setting an enclosure power cap.
For example, consider an HP BladeSystem c7000 enclosure fully configured with 16 server blades. Provisioning the
power to the total of the HP 2250W power supplies’ specification requires delivering 7836 watts to the enclosure.
You would typically use a single 30-amp 3-phase 208-volt circuit, which has a total capacity of 8640 watts.
However, if the enclosure’s peak power consumption over time is less than 4000 watts, you can provision and
operate two full enclosures from the same circuit with no performance loss, if you cap both enclosures at 4000-watts
(Figure 12).

20
Figure 12. Provisioning with Enclosure Dynamic Power Capping
Note that the group control algorithms used for Enclosure Dynamic Power Capping compensate for variables such
as hot-add disk drives and changing fan power, so no additional guardband is required.
The white paper “Dynamic Power Capping TCO and Best Practices” provides more detail on using Dynamic Power
Capping and Enclosure Dynamic Power Capping to manage data center power provisioning.
Additional uses for power capping
You can use both basic Power Capping and Dynamic Power Capping in other ways in the data center, including
emergency management and automatic power control during peak demand periods.
Power capping for emergency management
You can use power capping to manage server power consumption effectively when unforeseen circumstances arise.
For example, if a cooling system fails in part of a data center, you can manually lower the power cap on a group of
servers using Insight Control and power capping. This will quickly lower server power consumption and heat
generation in the affected area until you can restore cooling. Under these circumstances, it may be appropriate to
set power caps significantly lower, even though they may affect performance. Using HP SIM, you can define and
store groups of servers as collections. This allows you to apply power caps to these groups quickly in an
emergency.
Similarly, if all or part of a data center loses primary AC power, you can immediately apply a group power cap to
lower power consumption for server groups. This reduces the power drain on data center Uninterruptible Power
Supplies (UPSs), which increases the maximum window of time that the data center can remain operational after a
power failure but before the on-site generators restore power and cooling.
0 Watts
4000 Watts
(Measured Peak)
7836 Watts
(PSU Specification)
P ower P rovisioned to P S U
• 2612 x 3 = 7836 watts
• S ingle 3ØL ine = 8640
• 16 B lades per 8KW
Power Provisioned to Cap at
Measured Peak
•≈4KW enclosure
•32 Blades per 8KW
Power Provisioning with
Enclosure Dynamic Power Capping