Applies to: @RISK for Excel 5.x and Up

 

Question:
How much memory is used during a simulation in @RISK?

Answer:
@RISK saves the values of each output, each input (unless the default is changed on the Sampling tab of Simulation Settings), and each cell referred to by statistics functions such as RiskMean() or RiskPtoX().

• Memory per value: 8 bytes per iteration per simulation.

• Paging: To avoid overflowing 32-bit Excel’s limited memory, @RISK pages data to disk as needed.

Estimating overall memory usage:

1. In Simulation Settings → General, set Multiple CPU to Disabled.

2. Run a simulation with the same number of iterations.

3. While the progress window shows iterations:

   • Open Task Manager (Ctrl+Shift+Esc).

   • On Windows 7/Vista/XP: Look in the Processes tab, column Commit Size (enable via View → Select Columns if needed).

   • On Windows 8/10: Look in the Details tab, column Commit Size (enable via right-click column header → Select Columns).

4. Stop the simulation.

5. When Multiple CPU is re-enabled, the master CPU will use about this much memory; each worker CPU will use slightly less.

Smart Sensitivity Analysis and memory:

• Disabling Smart Sensitivity Analysis slightly speeds up simulations but generally does not reduce memory usage because precedent tracing memory is freed after tracing.

• For very large models, turning off Smart Sensitivity Analysis may avoid out-of-memory errors during tracing.

Excel memory limits:

• 64-bit Excel: effectively no limit; @RISK shares this memory space.

• 32-bit Excel: traditionally limited to 2 GB per process; updates to Excel 2013/2016 allow up to 4 GB on 64-bit Windows, 3 GB on 32-bit Windows.

• @RISK 7.5.1 and Up: parts sharing Excel memory are Large Address Aware.

• @RISK 7.5.2 and Up: all parts are Large Address Aware.

• All versions page data to disk to avoid exceeding Excel memory.

Multiple CPUs and memory:

• Each Excel process has its own memory limit; for 32-bit Excel, the total can be up to 2–4 GB × number of processors.

• Additional memory is used by executables like Pal* or Risk*.

• To limit CPU usage, see CPUs Used by @RISK.

Practical considerations:

• Not all RAM is available; the OS and other applications also consume memory.

• Ensure enough virtual memory is allocated. Task Manager can help monitor usage.

64-bit Excel benefits:

• Most simulations work fine in 32-bit Excel/@RISK.

• Simulations generating gigabytes of data may benefit from 64-bit Excel if sufficient RAM is available.

 

Related Articles:

"Out of Memory"

 "Not enough memory to run simulation"