PowerShell Performance Tips Guy Leech 

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@guyrleech Guy Leech • Freelance consultant writing PowerShell for software vendors like ControlUp, Flexxible, Avanite, Netwrix, Parallels (Alludo) • Wrote my first code (on Commodore PET) in 1980 (in BASIC) • Unix developer in C/C++ for 6 years after graduation • Started with Windows (NT 3.51) in 1995 via Citrix • Invented & wrote what is now Ivanti Application Control security product 

@guyrleech Loops  Don't keep evaluating/fetching/setting the same thing  Do you need Write-Verbose, Write-Progress etc on every iteration?  .Where (collections) versus Where-Object  ForEach versus ForEach-Object versus .ForEach (collections)  . { Process { } }  Start-Sleep – consider the impact of the script on your environment  Reduce the process' base priority  (Get-Process –id $pid).PriorityClass = 'BelowNormal' 

@guyrleech +=  For arrays and strings, takes a complete copy of existing object  Can get slow for large items  It is ok for hashtables  Arrays  Assign results to an array rather than adding to it within the loop  Wrap in @( ) to avoid PowerShell's "helpful" array flattening "feature"  Instead use System.Collections.Generic.List[object] which has Add/Remove methods  Iterate backwards if calling Remove()  Do not use System.Collections.ArrayList  Strings  $oldstring = $oldstring + $newstring  $oldstring = "$oldstring$newstring"  $oldstring = -join ( $oldstring , $newstring ) 

@guyrleech Caching Data  Is it quicker/cheaper/safer to keep in memory than look up individually/once?  Get-ADUser  Get-BrokerMachine  But what is the memory overhead?  Be selective what properties are cached  Trim own working set  Destroy unused data/variables  [gc]::Collect()  Hashtable or array ?  Unique key ?  Hashtable of arrays  Sort ?  Most likely to be accessed near the start  Binary search via index ?  Number of elements  .Where() versus Where-Object  .Where takes optional count x, for where-object pipe through Select-Object –First x  [System.Management.Automation.WhereOperatorSelectionMode]  .Where always returns a collection 

@guyrleech Caching Commands How long does it take PowerShell to find a command? Always use Import-Module in scripts in case of new/non- persistent profile In a loop, it can add time locating command Could module qualify (also helps avoid ambiguity/incorrect cmdlet use)  VMware.VimAutomation.Core\Get-VM versus Hyper-V\Get-VM Get-Command Call with . or & 

@guyrleech Parallelisation Can the tasks be split into separate, unrelated, activities? Pwsh 7.x has ForEach-Object –Parallel  3rd party module available for Windows PowerShell Windows PowerShell has Invoke-Command –ComputerName  Can use localhost  Needs remoting configured Jobs Runspaces  Not as scary as you might think!  Take some working code (give credit) and modify 

@guyrleech But Is Performance Important? Not usually Nowhere near as important as (correct) functionality Does taking 15 minutes instead of 5 really matter? How much time/money will it take to make it significantly faster? Do not run scripts on key infrastructure servers use a jump box Think about performance when writing code Expressions evaluated left to right in an if statement Filter as far left as possible 

@guyrleech Measuring Performance Measure-Command Once is not enough Watch for file system caching and other optimisations Get-Date [Diagnostics.Stopwatch] Guy's clock/stopwatch PSProfiler 

@guyrleech References https://gitgub.com/guyrleech https://github.com/IISResetMe/PSProfiler https://fatherjack.github.io/articles/Where https://4sysops.com/archives/understanding- powershell-begin-process-and-end-blocks/ https://www.youtube.com/c/GuyLeech42 

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