White Paper
Intel® Xeon® Processor
Processor Architecture
Analysis

Measuring Processor
Power
TDP vs. ACP
Specifications for the power a microprocessor can consume and dissipate can becomplicated and may vary from one manufacture to the other. When comparing power
specifications from one manufacture to the other it is important the comparisons are madewith specifications which are most similar to each other.
This paper specifically compares how Intel and AMD* specify processor power for theirserver processors and how you can use those specifications to more accurately compare
processors from each manufacturer.

April 2011
Revision: 1.1

White Paper Measuring Processor Power: TDP vs. ACP

Introduction

Intel has traditionally specified processor power with a specification called Thermal Design Power (TDP).
AMD also has a TDP specification. Recently, AMD has introduced an additional power value called ACP(Average CPU Power). As of this writing, ACP is only specified on AMD’s server processors. This white
paper examines the power specifications from both manufactures to help customers understand the best
way to compare them and to clarify the differences.
Let’s start with a definition of the two key power specifications, and then look at how these specifications
are typically used.

TDP (Thermal Design Power)Intel defines TDP as follows: The upper point of the thermal profile consists of the Thermal Design
Power (TDP) and the associated Tcase value. Thermal Design Power (TDP) should be used forprocessor thermal solution design targets. TDP is not the maximum power that the processor candissipate. TDP is measured at maximum TCASE.1. The thermal profile must be adhered to to ensureIntel’s reliability requirements are met. Note: Different processors SKU’s have different TDP’s. At thetime of this writing, Intel® Xeon® processors for 2 socket servers (5600 series) are available with a
TDP specification from 40W up to 130W depending on the particular SKU1.
AMD Opteron* processors also have a TDP specification. According to AMD documentation2 their TDP
specification is as follows:“TDP. Thermal Design Power. The thermal design power is the maximum power a processor can draw
for a thermally significant period while running commercially useful software. The constraining
conditions for TDP are specified in the notes in the thermal and power tables.”Notes:- TDP is measured under the conditions of all cores operating at CPU COF, Tcase Max, and VDD atthe voltage requested by the processor. TDP includes all power dissipated on-die from VDD,VDDNB, VDDIO, VLDT, VTT and VDDA.
- The processor thermal solution should be designed to accommodate thermal design power (TDP) at
Tcase,max.TDP is not the maximum power of the processor.

And from the same document, current AMD Opteron* processors have TDP specifications which range
from 35W up to 140W depending on the particular SKU.
A couple of additional comments about TDP specifications that applies to Intel® Xeon® processors,and most should also apply to AMD’s Opteron* processors:- Due to normal manufacturing variations, the exact thermal characteristics of each individual processor
are unique. Within the specified parameters of the part, some processors may operate at a slightly
higher or lower voltage, some may dissipate slightly higher or lower power and some may draw slightly
higher or lower current. As such, no two parts have identical power and thermal characteristics.
However the TDP specifications represent a “will not exceed” value.
- Because TDP is a worst case value when running a “worst case” application, most processors, whenrunning a more “typical” workload, will dissipate power that is less than the rated TDP value; how muchless will depend on the application and the specific part being tested. This is true not only for theprocessor, but for every electrical component in a server.
- It is important to note that thermal design power is the maximum thermal power the processor will
dissipate, but not the same as the maximum power the processor can consume. It is possible for theprocessor to consume more than the TDP power for a short period of time that isn’t “thermallysignificant”. For example, a processor might consume slightly more power than the rated TDP value forsay one microsecond…but then consume less power than the rated TDP value for a long period of time.

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White Paper Measuring Processor Power: TDP vs. ACP

Such operation is considered normal. Because the processor temperature does not exceed thespecified limits during such a short excursion, the processor will continue to operate correctly.
- It is possible to write “virus-like” code that toggles transistors in the processor on and off, but doesn’t
do any real work. Such “virus-like” code could cause the processor to exceed the rated TDP value for amuch longer, “thermally significant” period of time. To ensure the processor stays within the thermal
specification under such “virus-like” type conditions, Intel processors have a built-in “thermal control
circuit” which reduces processor power by reducing the processor voltage and/or modulates the clock
frequency.

What is AMD’s ACP?
According to AMD documentation3, ACP (Average CPU Power) is the average (Geometric Mean)
power a processor was measured to dissipate while running a collection of 5 different benchmarks(Transaction Processing Performance Council (TPC Benchmark*-C), SPECcpu*2006, SPECjbb*2005,
and STREAM.) It is also noted in the AMD documentation these measurements to determine the ACP
value are not done on every processor, but only on a particular processor that was selected from the“hotter side” of their manufacturing distribution.
Similar to the normal manufacturing variances that may result in a processor consuming less than therate TDP value, it is expected that each AMD processor will have a different actual ACP value. As ofthis writing, there are AMD Opteron* processors (6100 and 4100 series) with ACP values that range
from 32W up to 105W.
Although AMD’s thermal specification document2 does not mention ACP, based on earlier AMD
documentation4, we believe the tables below accurately reflects the TDP value of a processor with agiven ACP value. We believe the table on the left represents previous generation “Istanbul” processors,
and the table on the right represents current generation “Magny Cours” processors.

Previous Generation AMD
Opteron Processors (Istanbul)4
ACP
TDP
105 W
137 W
75 W
115 W
55 W
79 W
40 W
60 W

Current Generation AMD Opteron Processors(Magny Cours)2,3
ACP
TDP
CPU Series
105 W
140 W
6100 (G34)
80 W
115 W
6100 (G34)
65 W
85 W
6100 (G34)
75 W
95 W
4100 (C32)
50 W
65 W
4100 (C32)
32 W
35 W
4100 (C32)

Table 1: “Istanbul” TDP & ACP Values

Table 2: “Magny Cours” TDP & ACP Values

From AMD’s description, we conclude the ACP value is not a guaranteed specification and some parts
may have ACP values that are higher or lower than the ACP value quoted by AMD for a given SKU.
Also, AMD only quotes an ACP value for the Opteron* class of processors and does not quote ACP
values for any of their other processor lines.

* Other names and brands may be claimed as the property of others.

3White Paper Measuring Processor Power: TDP vs. ACPWhat thermal specifications are most useful?
If you are a thermal engineer, the processor TDP specification is very important because your thermal
solution (fans, heat sink, etc.) must be able to be able to dissipate the rated TDP value. Intel and AMD
both agree on this point. If a thermal engineer designs to ACP, it is likely the design will be undersize
and not keep the processor within its thermal specifications.
If you are a server customer, you are probably not concerned about how much power any given
component dissipates, but instead the power of the entire server when running your workload. To
measure actual server power consumption, you can easily connect a power meter to the input cord(s)of you server while running a variety of workloads.
ACP, while an interesting concept, does not help the thermal engineer design heat sinks; nor is it thebest method to determine the power of their server while running a given workload. But you may ask,
doesn’t ACP represent the power dissipated by a more “typical” workload, instead of a worst caseworkload? Yes it does, but there are still flaws in using ACP to estimate “real world” processor andserver power:
1. Since ACP is not measured on every processor, the actual power a given AMD processor will
dissipate may be higher or lower than the ACP value. TDP values, on the other hand, are a
specified value and are measured on every device that ships from the factory.
2. The workloads used to generate the ACP vale may not match the workload(s) of your specificuse.
3. The processor isn’t the only component in the server. Even if the ACP value is “close enough”
to estimate the power of the processor, no other component in the server has an equivalent
ACP value. And even if other components did have “ACP like” values it would be tedious tocollect those values and add them up to estimate the system power under “real world”
conditions.
4. It is very easy to measure the actual server power while running your specific workload. Just
plug your server into a power meter while running your specific application(s). So there is noneed to use a processor ACP value. You can simply measure system power which is moreaccurate and less effort than item #3 above.

ACP does not equal TDP
A common misuse of specs is to assume that TDP and ACP are similar specifications. As shown
above, TPD and ACP have very different definitions and different uses, and cannot be compared.
As an example, below are the SPECpower_ssj2008* results of two similar 2-socket servers from
Hewlett Packard, one with Intel processors and the other with AMD processors. Note the ACP value for
the AMD processors is 80W, while the TDP specification for the Intel processors is 95W. The
discrepancy in ACP vs. TDP power values might suggest the AMD platform should dissipate ~30W
lower system power. Yet according to the SPECpower results, the Intel platform is actually 49W lower
in overall system power at 100% target load. Thus the lower ACP value does not translate to a lower
system power.
In fact, if you look at the difference in processor TDP between these two platforms (180W for the Intelbased platform and 230W for the AMD-based platform) the 50W difference in TDP values for theseplatforms is nearly identical to the 49W system power difference measured at 100% target load. This
would suggest comparing processor TDP’s is more accurate at predicting relative platform power.

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White Paper Measuring Processor Power: TDP vs. ACP

Server A
Server B

Vendor/Server

Processor

Processor TDP

2 socket HP*
ProLiant* DL380 G7
2 socket HP ProLiant
DL385 G7

2 x Intel® Xeon®
processor X5675 3.07GHz
2 x AMD Opteronprocessor 6174 2.2GHz

2 x 95W = 180W
2 x 115W = 230W

Processor
ACP
N/A
2 x 80W =160W

Table 3: System Configuration Comparison for SPECpower_ssj2008

Server A
Server B

Performance1

System Power1

Performance/Watt2(higher is better)(Lower is better)(Higher is better)

894,314888,819

222W
271W

4020
3281

Table 4: SPECpower_ssj2008* Results
1.
2.

Performance and power measured in ssj_ops and watts @ 100% target load while running SPECpower_ssj2008
Performance to Power Ratio at 100% target load per SPECpower_ssj2008 resultsServer A used 222W@100% target load compared to Server B's 271W@100% target load. Server A has a
SPECpower_ssj2008 result of 3,197 overall ssj_ops/Watt and Server B has a SPECpower_ssj2008 result of 2,355
overall ssj_ops/Watt.
Data Source for Server A: (as of March 30, 2011)
http://www.spec.org/power_ssj2008/results/res2011q1/power_ssj2008-20110209-00353.html
Data Source for Server B: (as of March 30, 2011)
http://www.spec.org/power_ssj2008/results/res2010q2/power_ssj2008-20100323-00242.html

Bottom line: TDP and ACP are not equal. If you want to compare “apples to apples” it is recommend
you compare Intel’s TDP specification with AMD’s TDP specification. AMD’s ACP value has no equalfrom Intel. In addition, it is expected that both Intel and AMD’s processor will typically dissipate lessthan the rated TDP value when running real world applications.

Common misuses of processor power specifications
Max Voltage * Max Current = Max PowerAs noted above, the power, thermal and current specifications are used by specific engineers for
specific purposes. A common misuse of the specifications is to multiply the maximum current
specification, by the maximum voltage specification and determine the maximum power the processor
will dissipate. While the below equation is true:
Voltage x Current = PowerSuch a calculation does not mean a processor can or will dissipate this much power. The actual
voltage and current values of a processor are managed together to ensure the processor power
remains below the rated TDP value. If a given processor is running near its maximum current value, forexample, the voltage for this processor is likely well below the maximum voltage value.
Lower Power is Better
There are many press articles which assume a lower power server is also more energy efficient. This isactually far from the truth. Power by itself is not a measurement of overall server efficiency.
Performance of the server, in conjunction with the power consumed is what defines energy efficiency.
A system which is lower power, but is also lower performance will take longer to perform a task, and* Other names and brands may be claimed as the property of others.

5White Paper Measuring Processor Power: TDP vs. ACP

may ultimately consume more energy. The most efficient server is one which has the best performance
per watt. As an example, consider these two servers with the following performance and power
specifications:Server A
Server B

Vendor/Server
2 socket HP ProLiantDL380 G7
2 socket ZT Systems
1253Ra Datacener
Server*

Processor
2 x Intel® Xeon® processor X5675
3.07GHz
2 x AMD Opteron* processor
4164EE 1.8GHz

Processor TDP
2 x 95W = 180W
2 x 35W = 70W

Table 5: System Configuration Comparison for SPECpower_ssj2008
At first glance you might think Server B will be more efficient because it has lower total processor TDP(70W vs. 180W). However, when you look at the overall server performance per watt, you see a
different story. Server A has 218% higher performance, yet server power is only 176% greater thanServer B. Even though Server A does consume more power, it is 23.6% more energy efficient as shownby its better performance/watt.

System A
System B

Performance1

System Power1

Performance/Watt2(higher is better)(Lower is better)(Higher is better)

894,314
409,496

222W
126W

4020
3253

Table 6: SPECpower_ssj2008* Results
1.
2.

Performance and power measured in ssj_ops and watts @ 100% target load while running SPECpower_ssj2008
Performance to Power Ratio at 100% target load per SPECpower_ssj2008 resultsServer A used 222W@100% target load compared to Server B's 126W@100% target load. Server A has a
SPECpower_ssj2008 result of 3,197 overall ssj_ops/Watt and Server B has a result of 2,106 overall ssj_ops/Watt.
Data Source for Server A: (as of March 30, 2011)
http://www.spec.org/power_ssj2008/results/res2011q1/power_ssj2008-20110209-00353.html
Data Source for Server B: (as of March 30, 2011)
http://www.spec.org/power_ssj2008/results/res2010q2/power_ssj2008-20100601-00265.html

How should an end user measure/estimate “real world” server
power?
It is reasonable end users would want to know the power their servers are expected to consume whenrunning “real world” workloads. Estimating server power based on a worst case TDP specifications will
result in over estimating server power. Intel and AMD both agree on this point: “It is of little value tomeasure power consumption by only looking at the spec sheets for different components, adding thetotals together, because these generally only report the maximum power consumption.3” As such, an
ACP like value seems reasonable at first glance. However ACP only gives you the power of theprocessors when running “real world” applications. It doesn’t help estimating the power dissipated by
the other components in the server such as memory, hard drives, I/O boards, disk controllers etc.
All is not lost, however, because there is a very accurate way to measure server power under a users“real world” conditions. And again, Intel and AMD both agree on this point: “The best way to measurea server’s power consumption is the power meter, an inexpensive tool that is plugged into the wall, andthen your device, like a server, can be plugged into the power meter. The meter displays the wattage
drawn “at the wall” and allows you to analyze the power consumption under a variety of different
utilization levels.”3 So if an accurate “real world” power value is needed, simply measure it with a power
meter. Because of normal component power consumption tolerances, it is recommended that more

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White Paper Measuring Processor Power: TDP vs. ACP

than one sever is measured. Power can vary slightly even between identically configured servers fromthe same vendor.

Why doesn’t Intel provide an ACP value for their processors?
Intel sees no value adding another specification to our processors. As noted above, ACP is not usefulfor system or processor thermal engineers and end users can get more accurate power consumption
values by simply measuring the actual power of their server while running their specific application. In
addition, AMD does not specify exactly where in their silicon process distribution they measure ACP, so
it would be impossible for Intel to create an identical ACP specification.

Conclusion
While you can come to your own conclusions on the value and purpose of ACP, we want to convey thefollowing:- ACP does not equal TDP. These specifications are quite different and are not comparable.
- When comparing processor power specifications, we believe comparing TDP to TDP is the most“apples-to-apples’ comparison that can be made.
- If you want to determine the power of your server while running a “real world” workload, plug the best
and most accurate is to plug your server into a power meter and measure system power at the wall.

Sources:
1.

Intel® Xeon® Processor 5600 Series Datasheet Volume 1
http://www.intel.com/Assets/en_US/PDF/datasheet/323369.pdf (March 2010)
2. AMD Family 10h Server and Workstation Processor Power and Thermal Data Sheet.
http://support.amd.com/us/Processor_TechDocs/43374.pdf (June 2010)
3. http://www.amd.com/us/Documents/43761D-ACP_PowerConsumption.pdf (2010)
4. http://www.amd.com/us/Documents/43761C_ACP_WP_EE.pdf

AuthorScott Huck is a Performance Architect in Intel’s Data Center Group* Other names and brands may be claimed as the property of others.

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White Paper Measuring Processor Power: TDP vs. ACP
Software and workloads used in performance tests may have been optimized for performance only on Intel microprocessors.
Performance tests, such as SYSmark and MobileMark, are measured using specific computer systems, components, software,
operations and functions. Any change to any of those factors may cause the results to vary. You should consult other information andperformance tests to assist you in fully evaluating your contemplated purchases, including the performance of that product whencombined with other products.
Intel does not control or audit the design or implementation of third party benchmarks or Web sites referenced in this document. Intelencourages all of its customers to visit the referenced Web sites or others where similar performance benchmarks are reported andconfirm whether the referenced benchmarks are accurate and reflect performance of servers available for purchase.
This paper is for informational purposes only. THIS DOCUMENT IS PROVIDED “AS IS” WITH NO WARRANTIES WHATSOEVER. INCLUDINGWARRANTY OF MERCHANTABILITY, NONINFRINGEMENT, FITNESS FOR ANY PARTICULAR PURPOSE OR ANY WARRANTY OTHERWISE
ARISING OUT OF ANY PROPOSAL, SPECIFICATION OR SAMPLE. Intel disclaims all liability, including liability for infringement of any
proprietary rights relating to use of information in this specification. No license, express or implied, by estoppel or otherwise, to any
intellectual property rights is granted herein.
SPEC and the benchmark name SPECpower_ssj are trademarks of the Standard Performance Evaluation Corporation. Benchmark results
stated above reflect results published on http://www.spec.org as of November 22, 2010. For the latest SPECpower_ssj2008
benchmark results, visit http://www.spec.org/power_ssj2008/results/power_ssj2008.html.
Intel, the Intel logo, Intel®, Xeon®, and the Xeon® logos are trademarks or registered trademarks of Intel Corporation or its subsidiaries
in other countries. * Other names and brands may be claimed as the property of others. All timeframes, dates and products are
subject to change without further notification.
Copyright® 2011 Intel Corporation. All rights reserved.

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