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9.4. CPUs Used by @RISK 7.x
Applies to: @RISK 7.x
(If you have an older @RISK, see CPUs Used by @RISK 4.x–6.x.)
How many CPUs (cores or processors) do @RISK and RISKOptimizer use?
When you click Start Simulation, by default @RISK estimates how long a simulation will take and uses one or more CPUs to complete the simulation as quickly as possible.
- @RISK 7.x Industrial will use anywhere between one core and all the cores in your computer, depending on its estimate of the tradeoff between overhead of starting and managing multiple copies of Excel versus the savings from parallel processing.
- @RISK 7.x Standard and Professional will use no more than two cores, no matter how many you have.
- Simulations and optimizations with RISKOptimizer 7.5 use multiple cores, This lets the optimization run multiple simulations in parallel, to make progress faster.
- Simulations and optimizations with RISKOptimizer 7.0 use only one CPU.
- Simulations with Project use only one CPU.
If for any reason you want to limit @RISK to only one core when simulating or optimizing this workbook, open Simulation Settings and, on the General tab, change Multiple CPU to Disabled.
@RISK recognizes as a "CPU" anything that Windows recognizes as a CPU. To find the number of CPUs in your computer, press Ctrl-Shift-Esc to open Task Manager, then select the Performance tab. Real CPUs should make a major improvement in speed of large simulations, but hyperthreaded CPUs will give only modest speed improvement.
Multithreading, as opposed to multiple CPUs, is an Excel option, and you should generally turn it on in any edition of @RISK. See For Faster Simulations and Recommended Option Settings for Excel.
Can I limit the number used by @RISK, thus leaving some CPUs (cores) available for other programs? If @RISK decides to use only some cores, can I tell it to use more?
The default simulation setting of Multiple CPU — "Automatic" beginning with 7.5, "Enabled" in 7.0 — tells @RISK to decide the optimum number of CPUs. To tell @RISK to use only one CPU when simulating this workbook, go into Simulation Settings and change Multiple CPU Support to Disabled. To specify a number of CPUs greater than 1, the mechanism is different between @RISK 7.5 and @RISK 7.0.
Number of CPUs in @RISK 7.5 and newer:
Click Simulation Settings. On the General tab, look at the third setting, Multiple CPU Support. You have three options:
- The default is "Automatic" (equivalent to "Enabled" from earlier releases of @RISK). @RISK will decide how many cores to use — between 1 and the number in your computer in @RISK Industrial, 1 or 2 cores in @RISK Professional and @RISK Standard.
- "Enabled" has a new meaning. You specify a number in the #CPUs box, and then @RISK will always create that number of copies of Excel, even if the number is greater than the number of cores on your computer. If you use this setting, don't specify a number so high that the extra Excels bog down your computer. (Regardless of the number you specify, @RISK Professional and Standard won't create more than one "worker" Excel, for a total of two.)
- "Disabled" means that @RISK always uses just one core.
In earlier releases of @RISK, your setting for Multiple CPU Support applied only to simulations. Beginning with @RISK 7.5, it also applies to optimizations with RISKOptimizer.
The System Registry values RiskUseMultipleCores, ForceMultiCore, and NumCPU, and the Excel name _AtRisk_SimSetting_MaxCores, are no longer used in @RISK 7.5, and will be ignored if they are set.
Number of CPUs in @RISK 7.0:
To tell @RISK to use a certain number of CPUs, define a workbook-level name, RiskUseMultipleCores.
On Excel's Formulas tab, click Name Manager. If the name RiskUseMultipleCores already exists, click it and click Edit; otherwise click New and enter that name. The value can be any of the following:
- A specific number of cores that you want @RISK to use. If you specify more than the computer has, @RISK will use as many as you have but won't display an error message. If you specify a number greater than 2 in @RISK Professional or Standard, @RISK will use two cores but won't display an error message.
- The keyword all.
- The keyword off (equivalent to 1).
- The keyword auto (tells @RISK to decide the optimum number of CPUs).
- An absolute cell reference with leading equal sign, such as =$B$12. This lets you place the setting in the workbook in case you want to change it later without going through Name Manager, for instance if you're testing simulation speed with various numbers of cores.
If you define the name RiskUseMultipleCores in a workbook, it overrides the Multiple CPU setting (Enabled or Disabled) in Simulation Settings when that workbook is open.
The System Registry values ForceMultiCore and NumCPU, and the Excel name _AtRisk_SimSetting_MaxCores, are no longer used in @RISK 7.0, and will be ignored if they are set.
Additional keywords: Number of cores, multiple cores, how many cores, how many CPUs
Last edited: 2017-10-05