Uncovering the Power in Power Calculators

Jacques Bessoudo, Systems Technical Marketing

There has been much confusion over what power calculators are and I have run into differing opinions about what they should be or should do. Some expect power calculators to be datacenter planning tools, others expect them to be a guideline of how much power a system will consume.

Both are valid expectations, but when using a specific tool, no matter which vendor, it is of the utmost importance to understand what it provides: this should be specified by the vendor that publishes the calculator in order to disclose the intention of the calculator.

What They Are

Power calculators are tools that provide information about the power consumption of a system. They come in many different forms and each vendor has its own way of providing the data. Until recently, there was no industry standard benchmark for power consumption, so vendors did what they thought was right in order to provide useful information to customers - of course, most vendors didn't agree on what was useful to the customer, so most power calculators available on websites provide data that can't be compared across vendors because they are based on different assumptions and workloads. Below is a summary that indicates where these power calculators can be found and what they use to obtain the data.

     \* Workload: SPECjbb, SPEChpc
     \* Where to find it: Online or downloadable calculator
HP blades
     \* Workload: Undisclosed - see article
     \* Where to find it: Online or downloadable calculator
HP rack
     \* Workload: Undisclosed - see article
     \* Where to find it: Downloadable calculator
     \* Workload: Prime95
     \* Where to find it: Downloadable calculator
     \* Workload: SPECjbb
     \* Where to find it: Online calculators

Ideally, the data in the power calculators is obtained from running a workload in a real system and measured at the power inlet - between the wall and the power supplies. It is not modeled or inferred from product specifications, but measured data using instrumentation and real systems.

What They are NOT

Power calculators, unless specifically stated, are not datacenter planning tools. The reason for this is that not all workloads have the same behavior. The power calculator will provide a specific reference, but if the workload is any different, then the power consumption characteristics will change. Even environment variables like the room's air density, humidity and temperature will affect the results of these measurements.

System documentation typically includes a datacenter planning guide that specifies what needs to be provided for the correct operation of the system. In most cases, the data included in that document can also be found in the power supply or system label; if there is no datacenter planning guide, this label may come in handy to determine how much power a system is likely to draw as a maximum, since power supplies are designed specifically for the systems they will be energizing.

What They Provide

Systems vendors (Dell, HP, IBM and Sun) provide in their calculators at least two data points - idle power consumption and max power consumption. Max power consumption is specific to the workload run on the platform. Idle is typically the power consumption of a server that is running an OS at the login prompt or that has been logged in with no workload running.

Some vendors provide additional information, like:

    \* More than one workload, such as SPECjbb and/or SPEChpc and/or Linpack on specific products
    \* BTU's/hr to help calculate the air conditioning that will be required for that workload (this can be manually calculated by multiplying Watts by 3.41). BTUs/hr=3.41\*Watts
    \* Datacenter power requirements, which is typically higher than 'calculated power consumption'
    \* A slider or a % input box that allows the user to 'fine tune' the output of the calculator based on the expected utilization of the server
    \* The ability to configure a rack with the hardware and obtain total rack parameters
    \* Suggestions on the type of Power Distribution Units that will be necessary for the configurations in the rack

Things to be Aware of

Power calculator result comparisons

It is very tempting to take numbers from one vendor's power calculator and compare against another vendor's power calculator, but they don't work that way because systems are not running the same workload in the same environment. The closest you can get to compare data between calculators is to compare idle power and even then, the numbers aren't 100% apples-to-apples, because even the operating system running on these servers might not be the same.

On redundant power supplies

Power supplies in an enterprise class system can be configured in redundant mode for higher availability to prevent the system from shutting down in case of a power supply failure or a power grid failure - if the datacenter is equipped with redundant power grids.

Redundancy is very beneficial for the uptime of the server, but not always ideal for the power efficiency, because of the power curves of power supplies. Some power supplies don't hit reasonable power efficiencies until a high load is demanded from them; at Sun, high efficiency power supplies are prolific in the product line.

Not all power supplies are equal and each has it's own load / efficiency curve. Ideally, a power supply will reach a reasonable efficiency, say 80%, under reasonably small loads, say 20-30%. From this point on, as the load increases, efficiency should stay above 80% and ideally reach or surpass 90%.

This is a very important aspect to consider: a fully loaded system running all components at maximum will only achieve 50% of the power supply capacity because the load is balanced between the two power supplies. Power supplies that are not efficient from a small load will waste a lot more energy when they are idle, as the load on each of the power supplies might be as low as 15-30% of the total capacity.

Idle systems are inherently the most inefficient systems, because they are wasting energy without producing any work. The higher the utilization of a system, the more work it produces and the better the efficiency in all aspects - electrically efficient by design and [work]/watt whether 'work' is queries per minute or web transactions.

Measuring power on your own

The best way to figure the power consumption of a system is to run it in its actual environment and measure the power it consumes while running the typical application. In order to do this, the internal sensors of the servers can be of great use, since they provide power consumption information with reasonable accuracy. The data from these sensors is usually available from the ILOM remote management interface (depending on the system) under the System Monitoring > Power Management tab.

For rackmount servers, a simple meter like the ones available from Watt's-up can be very useful; their products include simple meters that provide the information on an LCD display, as well as more sophisticated ones that have serial ports for power monitoring, or even a web interface.

Yet another way to do this is to use the Real Time Power Monitoring and Management Service that was recently released for a limited set of products. It requires no hardware and only a small lightweight package to be installed on the servers to be monitored.


The components of which a calculator consists, could you perhaps give me the list,via the my email address?

Posted by Timothy pienaar on April 28, 2009 at 06:58 PM PDT #

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