HomeTroubleshooting@RISK for Excel: SimulationMultiple CPU -- Only One CPU Runs

4.5. Multiple CPU -- Only One CPU Runs

Applies to: @RISK 5.x–7.x

I have Simulation Settings set to Multiple CPU Enabled, and my computer does have multiple cores, but @RISK uses only one of them. The simulation progress window shows only one CPU; it's just as though @RISK only knows about one CPU on my computer.

@RISK uses a heuristic to guess how long a simulation will take. If @RISK judges that the overhead of starting multiple copies of Excel would outweigh the time saved through parallel processing, it will use only one core even if you have enabled Multiple CPU.

@RISK 6.2.1 and earlier were pretty liberal about making that decision, so a number of short simulations ran in multiple CPUs that actually would have been a bit faster with a single CPU. If you notice that situation, just go into Simulation Settings, change Multiple CPU to Disabled, and save your workbook.

My simulation seems to be taking a long time with a single CPU, and I think it would go faster using multiple CPUs.

First, make sure you have Multiple CPU set to Enabled, on the General tab of Simulation Settings. (This setting is stored in each workbook.) If you do, you can force multiple CPU for all simulations as explained in CPUs Used by @RISK 7.x or CPUs Used by @RISK 4.x–6.x.

My simulation with a project takes a long time, but only one CPU is used.

That is by design. Simulations with Microsoft Project can use only one CPU. However, if @RISK is using the standard engine, you may be able to speed up your simulation by using the accelerated engine. Follow this advice to see if @RISK can use the accelerated engine to simulate this project. (The accelerated engine still uses only one CPU, but on most projects it's significantly faster than the standard engine.)

My simulation used to run in multiple CPUs, but since I upgraded to @RISK 6.3 or newer it uses only one, and the simulation takes longer.

Beginning with 6.3.0, @RISK is more conservative in its estimate, so that some small simulations that used multiple CPUs in earlier versions of @RISK will now run a single CPU. @RISK makes this decision on the basis of how long an Excel recalculation takes. If it seems that your simulations are taking longer with a single CPU, see the System Registry edit in the preceding answer.

I have Simulation Settings set to Multiple CPU Enabled, and my computer does have multiple cores, but @RISK uses only one of them.  The simulation progress window shows all my CPUs, but only one of them actually runs iterations.  The others all say "Excel started".  Then at the end of simulation @RISK hangs and I have to kill those other copies of Excel in Task Manager.

Please check in Excel's disabled items list to see whether any items associated with @RISK have been disabled, particularly RiskSimMultiCpu6.xla or RiskSimMultiCpu7.xla.  (When Excel crashes, it sometimes marks every open add-in as disabled.  The add-in cannot run again until you specifically remove it from the disabled list.)

Last edited: 2016-04-15

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