Can Powershell Run Commands in Parallel? - multithreading

Question

I have a powershell script to do some batch processing on a bunch of images and I'd like to do some parallel processing. Powershell seems to have some background processing options such as start-job, wait-job, etc, but the only good resource I found for doing parallel work was writing the text of a script out and running those (PowerShell Multithreading)
Ideally, I'd like something akin to parallel foreach in .net 4.
Something pretty seemless like:
foreach-parallel -threads 4 ($file in (Get-ChildItem $dir))
{
.. Do Work
}
Maybe I'd be better off just dropping down to c#...

Answer 1

You can execute parallel jobs in Powershell 2 using Background Jobs. Check out Start-Job and the other job cmdlets.
# Loop through the server list
Get-Content "ServerList.txt" | %{
# Define what each job does
$ScriptBlock = {
param($pipelinePassIn)
Test-Path "\\$pipelinePassIn\c`$\Something"
Start-Sleep 60
}
# Execute the jobs in parallel
Start-Job $ScriptBlock -ArgumentList $_
}
Get-Job
# Wait for it all to complete
While (Get-Job -State "Running")
{
Start-Sleep 10
}
# Getting the information back from the jobs
Get-Job | Receive-Job

Answer 2

The answer from Steve Townsend is correct in theory but not in practice as #likwid pointed out. My revised code takes into account the job-context barrier--nothing crosses that barrier by default! The automatic $_ variable can thus be used in the loop but cannot be used directly within the script block because it is inside a separate context created by the job.
To pass variables from the parent context to the child context, use the -ArgumentList parameter on Start-Job to send it and use param inside the script block to receive it.
cls
# Send in two root directory names, one that exists and one that does not.
# Should then get a "True" and a "False" result out the end.
"temp", "foo" | %{
$ScriptBlock = {
# accept the loop variable across the job-context barrier
param($name)
# Show the loop variable has made it through!
Write-Host "[processing '$name' inside the job]"
# Execute a command
Test-Path "\$name"
# Just wait for a bit...
Start-Sleep 5
}
# Show the loop variable here is correct
Write-Host "processing $_..."
# pass the loop variable across the job-context barrier
Start-Job $ScriptBlock -ArgumentList $_
}
# Wait for all to complete
While (Get-Job -State "Running") { Start-Sleep 2 }
# Display output from all jobs
Get-Job | Receive-Job
# Cleanup
Remove-Job *
(I generally like to provide a reference to the PowerShell documentation as supporting evidence but, alas, my search has been fruitless. If you happen to know where context separation is documented, post a comment here to let me know!)

Answer 3

There's so many answers to this these days:
jobs (or threadjobs in PS 6/7 or the module for PS 5)
start-process
workflows (PS 5 only)
powershell api with another runspace
invoke-command with multiple computers, which can all be localhost (have to be admin)
multiple session (runspace) tabs in the ISE, or remote powershell ISE tabs
Powershell 7 has a foreach-object -parallel as an alternative for #4
Using start-threadjob in powershell 5.1. I wish this worked like I expect, but it doesn't:
# test-netconnection has a miserably long timeout
echo yahoo.com facebook.com |
start-threadjob { test-netconnection $input } | receive-job -wait -auto
WARNING: Name resolution of yahoo.com microsoft.com facebook.com failed
It works this way. Not quite as nice and foreach-object -parallel in powershell 7 but it'll do.
echo yahoo.com facebook.com |
% { $_ | start-threadjob { test-netconnection $input } } |
receive-job -wait -auto | ft -a
ComputerName RemotePort RemoteAddress PingSucceeded PingReplyDetails (RTT) TcpTestS
ucceeded
------------ ---------- ------------- ------------- ---------------------- --------
facebook.com 0 31.13.71.36 True 17 ms False
yahoo.com 0 98.137.11.163 True 97 ms False
Here's workflows with literally a foreach -parallel:
workflow work {
foreach -parallel ($i in 1..3) {
sleep 5
"$i done"
}
}
work
3 done
1 done
2 done
Or a workflow with a parallel block:
function sleepfor($time) { sleep $time; "sleepfor $time done"}
workflow work {
parallel {
sleepfor 3
sleepfor 2
sleepfor 1
}
'hi'
}
work
sleepfor 1 done
sleepfor 2 done
sleepfor 3 done
hi
Here's an api with runspaces example:
$a = [PowerShell]::Create().AddScript{sleep 5;'a done'}
$b = [PowerShell]::Create().AddScript{sleep 5;'b done'}
$c = [PowerShell]::Create().AddScript{sleep 5;'c done'}
$r1,$r2,$r3 = ($a,$b,$c).begininvoke() # run in background
$a.EndInvoke($r1); $b.EndInvoke($r2); $c.EndInvoke($r3) # wait
($a,$b,$c).streams.error # check for errors
($a,$b,$c).dispose() # clean
a done
b done
c done

Answer 4

In Powershell 7 you can use ForEach-Object -Parallel
$Message = "Output:"
Get-ChildItem $dir | ForEach-Object -Parallel {
"$using:Message $_"
} -ThrottleLimit 4

Answer 5

http://gallery.technet.microsoft.com/scriptcenter/Invoke-Async-Allows-you-to-83b0c9f0
i created an invoke-async which allows you do run multiple script blocks/cmdlets/functions at the same time. this is great for small jobs (subnet scan or wmi query against 100's of machines) because the overhead for creating a runspace vs the startup time of start-job is pretty drastic. It can be used like so.
with scriptblock,
$sb = [scriptblock] {param($system) gwmi win32_operatingsystem -ComputerName $system | select csname,caption}
$servers = Get-Content servers.txt
$rtn = Invoke-Async -Set $server -SetParam system -ScriptBlock $sb
just cmdlet/function
$servers = Get-Content servers.txt
$rtn = Invoke-Async -Set $servers -SetParam computername -Params #{count=1} -Cmdlet Test-Connection -ThreadCount 50

Answer 6

Backgrounds jobs are expensive to setup and are not reusable. PowerShell MVP Oisin Grehan
has a good example of PowerShell multi-threading.
(10/25/2010 site is down, but accessible via the Web Archive).
I'e used adapted Oisin script for use in a data loading routine here:
http://rsdd.codeplex.com/SourceControl/changeset/view/a6cd657ea2be#Invoke-RSDDThreaded.ps1

Answer 7

To complete previous answers, you can also use Wait-Job to wait for all jobs to complete:
For ($i=1; $i -le 3; $i++) {
$ScriptBlock = {
Param (
[string] [Parameter(Mandatory=$true)] $increment
)
Write-Host $increment
}
Start-Job $ScriptBlock -ArgumentList $i
}
Get-Job | Wait-Job | Receive-Job

Answer 8

If you're using latest cross platform powershell (which you should btw) https://github.com/powershell/powershell#get-powershell, you can add single & to run parallel scripts. (Use ; to run sequentially)
In my case I needed to run 2 npm scripts in parallel: npm run hotReload & npm run dev
You can also setup npm to use powershell for its scripts (by default it uses cmd on windows).
Run from project root folder: npm config set script-shell pwsh --userconfig ./.npmrc
and then use single npm script command: npm run start
"start":"npm run hotReload & npm run dev"

Answer 9

This has been answered thoroughly. Just want to post this method i have created based on Powershell-Jobs as a reference.
Jobs are passed on as a list of script-blocks. They can be parameterized.
Output of the jobs is color-coded and prefixed with a job-index (just like in a vs-build-process, as this will be used in a build)
Can be used to startup multiple servers at a time or running build steps in parallel or so..
function Start-Parallel {
param(
[ScriptBlock[]]
[Parameter(Position = 0)]
$ScriptBlock,
[Object[]]
[Alias("arguments")]
$parameters
)
$jobs = $ScriptBlock | ForEach-Object { Start-Job -ScriptBlock $_ -ArgumentList $parameters }
$colors = "Blue", "Red", "Cyan", "Green", "Magenta"
$colorCount = $colors.Length
try {
while (($jobs | Where-Object { $_.State -ieq "running" } | Measure-Object).Count -gt 0) {
$jobs | ForEach-Object { $i = 1 } {
$fgColor = $colors[($i - 1) % $colorCount]
$out = $_ | Receive-Job
$out = $out -split [System.Environment]::NewLine
$out | ForEach-Object {
Write-Host "$i> "-NoNewline -ForegroundColor $fgColor
Write-Host $_
}
$i++
}
}
} finally {
Write-Host "Stopping Parallel Jobs ..." -NoNewline
$jobs | Stop-Job
$jobs | Remove-Job -Force
Write-Host " done."
}
}
sample output:

Answer 10

There is a new built-in solution in PowerShell 7.0 Preview 3.
PowerShell ForEach-Object Parallel Feature
So you could do:
Get-ChildItem $dir | ForEach-Object -Parallel {
.. Do Work
$_ # this will be your file
}-ThrottleLimit 4