PowerShell Serial Function to Parallel Workflow

I’ve been feeling very underwhelmed about blogging for the past week, so I needed something short and simple to discuss this time. Luckily I had an old script that I could adapt and explain.

PowerShell introduced Workflows in version 3, and they are particularly wonderful for the ability to create a ForEach loop that runs in parallel. This can significantly reduce your run times when making numerous remote connections, but with most things, it depends on your use and environment.

A standard PowerShell function executes against multiple computers in a serial fashion, meaning one at a time. This is perfectly fine for a small number of computers, but as the list increases so does the time required. Adapting to a Workflow is not especially difficult, but you may run into some invalid commands. In my below examples, I built a function and workflow with nearly identical syntax. If you look closely though, you’ll see that the Get-Service cmdlet includes a -PSComputerName parameter instead of the standard -ComputerName parameter. Also, Out-Null was not included in Workflows. I had to pipe to InlineScript in order to get that to work. Further details about Workflow scripting can be found here.

The example functions execute serially while the workflow executes in parallel. Get-ServiceTimed utilizes a standard cmdlet, but also calculates the total run time like the rest of the examples. I’ve included it solely for comparison to Get-ServiceSerial which has a ForEach loop – so as to be as similar to the workflow as possible.

Each function calculates and returns the runtime.

Executed in 33.51 seconds against 32 computers.
Executed in 0.03 seconds against 2 computers.

Function Get-ServiceTimed
$StartTime = Get-Date
Get-Service -ComputerName $Computers | Out-Null
"Run Time - Cmdlet - $(((Get-Date) - $StartTime).TotalSeconds) seconds."

ForEach Serial
Executed in 29.30 seconds against 32 computers, just as slow as using the cmdlet normally.
Executed in 0.03 seconds against 2 computers.

Function Get-ServiceSerial
$StartTime = Get-Date
ForEach ($Computer In $Computers)
{ Get-Service -ComputerName $Computer | Out-Null }
"Run Time - Serial - $(((Get-Date) - $StartTime).TotalSeconds) seconds."

ForEach Parallel
Executed in 12.36 seconds against 32 computers. More than twice as fast as the serial execution.
Executed in 5.88 seconds against 2 computers. Significantly slower than any serial test.

WorkFlow Get-ServiceParallel
$StartTime = Get-Date
ForEach -Parallel ($Computer In $Computers)
{ Get-Service -PSComputerName $Computer | InlineScript { Out-Null } }
"Run Time - Parallel - $(((Get-Date) - $StartTime).TotalSeconds) seconds."

Summary I only supplied a few example execution times because times vary wildly depending on network traffic and configuration. Your results may vary greatly. Serial execution was actually faster than parallel execution on average until I was connecting to more than ten computers. If you only work with a handful of computers, you may not need to worry about this. I’m often connecting to at least 30 computers at once, in which case parallel execution is always much faster.


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