Saturday, May 16, 2009

Generating 100% CPU with calc or PowerShell

Occasionally when testing something I want to generate 100% CPU load on a Windows computer. There are several utilities out there to do this, but that implies you have the utility on hand and are comfortable running it on the server. A colleague of mine (thanks Mark S.) showed me this nifty trick of using calc.exe to generate 100% CPU. The best thing about this is that every standard Windows OS installation has calc.exe.

This post provides two methods of generating 100% CPU load, the original calc.exe method, and a simple one line PowerShell command to do the same thing from the command-line (locally or on a remote server with PS v1 and psexec). Note that there may be a better method with PowerShell, I simply scripted the same operation calc was performing.

On dual-CPU/core computers this uses 100% of one CPU/core. To use more than that, calc or the PowerShell command can be run more than once, and Windows by default will run the new process on another less-busy CPU/core.

One practical application of this is to load-test a VMware VI3 cluster, generating 100% CPU on one or more VMs to see how ESX and DRS/VC handles the load. I have also used this in the past when testing multi-threaded applications and processor affinity to see how Windows allocates a processor.


Use calc to calculate the factorial of a number - the product of all integers from 1 up to and including the number specified, eg 5! = 1x2x3x4x5

  1. Run calc.exe and switch to scientific mode
  2. Type a large number (eg. 12345678901234567890), press the 'n!' button.
  3. Calc will ask to confirm after warning this will take a very long time
  4. 100% CPU utilisation will now occur (essentially forever)
Using the largest int32 positive integer, calculate the factorial to generate 100% CPU utilisation
$result = 1; foreach ($number in 1..2147483647) {$result = $result * $number};

Depending on how fast the CPU is, this could finish, so a loop to run the command above 2 billion times:
foreach ($loopnumber in 1..2147483647) {$result=1;foreach ($number in 1..2147483647) {$result = $result * $number};$result}

If you want to see how long the command takes to run:
Measure-Command {$result = 1; foreach ($number in 1..2147483647) {$result = $result * $number}}

Using the command-line then provides the ability to run the command remotely. To use psexec to remotely execute powershell v1 factorial to generate 100% CPU:
psexec \\%computername% /s cmd /c "echo. | powershell $result = 1; foreach ($number in 1..2147483647) {$result = $result * $number}"

Wayne's World of IT (WWoIT), Copyright 2009 Wayne Martin. 


Anonymous said...

Hi Wayne, David B (SSA) here. Just thought I'd let you know that it looks like M$ have put a little trap in calc for W2k8 (R2 at least) that stops you doing this. PS option still works a treat though.

John Lucas said...

This seems to work somewhat, but for some reason won't push either core of my CPU beyond 50%. I'm guessing there is some sort of load limiter which is restricting how much CPU powershell can use?

Anonymous said...

Hi John - regarding the 50% CPU. When I ran I found the powershell process is not Multi-threaded hence sticks to 1 vCPU (or CORE). If you saw 50% cpu then you'd probably see 100% CPU on 1 core and ~0% CPU on the other hence average of 50%.

Just though I'd share since I had same experience.

Mark said...

I got around the 'this doesn't peg my processor' by running the same command several times in different windows. Each instance increased CPU utilization by 3.4%.

Anonymous said...

Just in case anybody is interested in getting all CPU cores busy:

# specify the number of cores
ForEach ($cores in 1..4){
start-job -ScriptBlock{
$result = 1;
foreach ($loopnumber in 1..2147483647)
foreach ($number in 1..2147483647)
$result = $result * $number

# toggle a breakpoint here to easyly stop jobs by pressing F5
Stop-Job *

Thomas Kipping said...


best regards
Thomas from computer 22

Anonymous said...

Building on previous script, can utilize WMI to get the number of Cores

$NumberOfLogicalProcessors = Get-WmiObject win32_processor | Select-Object -ExpandProperty NumberOfLogicalProcessors

ForEach ($core in 1..$NumberOfLogicalProcessors){

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About Me

I’ve worked in IT for over 13 years, and I know just about enough to realise that I don’t know very much.