PowerShell 7 Preview 4

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We continue to make progress towards our PowerShell 7 release which currently is targeting December 2019 for a Release Candidate and January 2020 for General Availability and will be our first LTS (Long Term Servicing) release!

Please see the previous blog post on Preview 3 for more details about LTS and also Windows PowerShell compatibility.

Preview 4 contains a number of bug fixes, but also new features which I’ll cover in this blog post.

New Features in Preview 4

This is just a small part of the entire changelog. New experimental features in this preview from the community and also the PowerShell team:

Ternary Operator

The ternary operator is popular among C# developers due to its terseness which can improve readability if you are familiar with this operator.

This operator is completely opt-in so if you prefer to use if..else instead, you can certainly continue to do that.

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Start-Job -WorkingDirectory

Those of you familiar with the Start-Job cmdlet will have encountered that the new PowerShell process started to handle the job will have different working directory on Windows PowerShell and PowerShell Core and it can sometimes be not what you expected. This parameter was added to allow you to specify the working directory of the new job process before your script block runs!

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$ErrorActionPreference = “Break”

This feature comes from a well known PowerShell MVP Kirk Munro. Basically, if you set $ErrorActionPreference to Break, then when there is an error it will drop you into the debugger immediately!

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Invoke-DscResource

With this change, you can now leverage DSC Resources while by-passing the LCM (Local Configuration Manager). This means that you can author your own LCM or simply leverage existing DSC Resources within your scripts and this also works cross platform!

Note that binary DSC Resources are not supported!

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DSC Configuration Compilation

Previously if you authored a DSC Configuration script, you would need to use a Windows machine to compile it to a mof file to deploy onto your managed node. Starting with Preview4, you can now perform DSC compilation on non-Windows systems.

Note that this is work in progress with some known issues.

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Testing the MSIX package

Recently, we started publishing a MSIX package for Windows. This will eventually allow us to publish PowerShell 7 to the Windows Store. For now, if you wish to try out this package, you must be in Developer Mode and use Add-AppxPackage to install it. Double clicking it from the Windows Shell will not allow you to install the developer signed package.

Closing

Although this blog post focuses on new features, this release also contains many bug fixes as well as targeted performance improvements.

You can always get the latest version of PowerShell from https://aka.ms/get-powershell.

Expect more new features from the community and the PowerShell team in future Preview releases!

Steve Lee
PowerShell Team

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Release of PowerShell Script Analyzer (PSScriptAnalyzer) 1.18.2

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In keeping with the tradition of releasing improvements to PSScriptAnalyzer more often, we’re happy to announce that 1.18.12 is now available! As a dependency of PowerShell Editor Services (a module used by editor extensions like the PowerShell Visual Studio Code extension), this release is motivated by a desire to further stabilize our editor experience. At the moment, the Visual Studio Code PowerShell extension still ships with PSScriptAnalyzer 1.18.0. After fixing some undesirable edge cases between 1.18.1 and 1.18.2, we intend to ship an update to the Visual Studio Code extension that will include 1.18.2.

The blocking issue that it resolves is quite technical and should not concern end-users, but for those who are interested: starting with1.18.1, a performance optimization was added whereby we started to share and cache a PowerShell runspace pool instead of creating a new one for every command invocation. However, it turns out that there is an edge case where, when dealing with specific commands from thePackageManagementmodule, the runspace pool can get into a deadlock, which causes the execution of PSScriptAnalyzer to hang indefinitely. This is due to a bug inPackageManagementitself (a very unfortunate asynchronous API call that leads to the deadlock) but also PowerShell itself, which should be able to handle bad scenarios like this. Therefore, a workaround for this had to be implemented in PSScriptAnalyzer by blacklisting the PackageManagement commands.

Given that the other changes in this release are mainly fixes and small enhancements, we decided to not bump the minor version number. We ask that the community participate in testing and giving feedback on this update before it ships by default in the Visual Studio Code extension. You can make this new update with the Visual Studio Code extension start by executing the following command:

Install-Module -Name PSScriptAnalyzer -Repository PSGallery -Scope CurrentUser

Should you find that there are changes that you are not happy with, please report them here.

Optionally, you can roll back to the default included version of PSScriptAnalyzer by running Uninstall-Module -Name PSScriptAnalyzer.

In this release, we’ve made the following fixes

  • PipelineIndentation: More edge cases when using non-default values of this setting (NoIndentation in the Visual Studio Code extension) were fixed. This feature was only introduced in1.18.0and we hope the be closer to a state now where we could potentially change the default.
  • New compatibility rule profiles were added for non-Windows OSs on PowerShell 7 (preview). Additionally, fixes were made to profile generation to support macOS and Linux.
  • A fix was made to PSCloseBrace to correctly flag the closing brace of a one-line hashtable, correcting some broken formatting.

Enhancements were made in the following areas

  • When using settings files, error messages are now much more actionable.
PS> Invoke-ScriptAnalyzer -Settings /tmp/MySettings.psd1 -ScriptDefinition 'gci'

Invoke-ScriptAnalyzer : includerule is not a valid key in the settings hashtable.
Valid keys are CustomRulePath, ExcludeRules, IncludeRules, IncludeDefaultRules,
RecurseCustomRulePath, Rules and Severity. 
...

  • PSScriptAnalyzer has a logo now thanks to the community member @adilio
  • The formatter was enhanced to also take commented lines into account in multi-line commands
  • The formatter was enhanced to optionally allow correction of aliases as well. With this change, a setting in the Visual Studio Code extension will soon be made available to configure this. By default, this setting will not be on for the moment. We are open to feedback: while there are very likely a few people that would love for it to be enabled, it may upset others.
  • UseDeclaredVarsMoreThanAssignmentsnow also takes into account the usage of Get-Variable with an array of variables and usage of the named parameter -Name

We’ve also made some changes in our GitHub repository and changed the default branch from development to master to simplify the development workflow and be consistent with other repositories in the PowerShell organization. If you have a fork of the project, you will need to make this change in your fork as well or remember to use master as a base and open pull requests against master. This also means that the next version of the Visual Studio Code extension will point tomasterfor the documentation of PSScriptAnalyzer’s rules.

The Changelog has more details if you want to dig further.

Future Directions

We are thinking of following an approach similar to the Visual Studio Code extension where we make a version 2.0 at that drops support for PowerShell version 3 and 4. One of the next changes could be to improve how PowerShellEditorServices calls into PSScriptAnalyzer: currently, Editor Services uses the PSScriptAnalyzer PowerShell cmdlets which means that we have to create an entire instance of PowerShell for these invocations. Knowing that bothPowerShellEditorServicesandPSScriptAnalyzerare binary .NET modules, we could directly call into PSScriptAnalyzer’s .NET code by publishing a NuGet package of PSScriptAnalyzer with suitable public APIs. Given that PSScriptAnalzyer currently performs a conditional compilation for each PowerShell version (3, 4, 5, and 6+), dropping support for version 4 and 5 could help make the aforementioned move to an API model much easier to implement. Please give feedback if your use case ofPSScriptAnalyzerwould be impacted by this.

On behalf of the Script Analyzer team,

Christoph Bergmeister, Project Maintainer from the community, BJSS
Jim Truher, Senior Software Engineer, Microsoft

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PowerShell ForEach-Object Parallel Feature

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PowerShell ForEach-Object Parallel Feature

PowerShell 7.0 Preview 3 is now available with a new ForEach-Object Parallel Experimental feature. This feature is a great new tool for parallelizing work, but like any tool, it has its uses and drawbacks.

This article describes this new feature, how it works, when to use it and when not to.

What is ForEach-Object -Parallel?

ForEach-Object -Parallel is a new parameter set added to the existing PowerShell ForEach cmdlet.

ForEach-Object -Parallel <scriptblock> [-InputObject <psobject>] [-ThrottleLimit <int>] [-TimeoutSeconds <int>] [-AsJob] [-WhatIf] [-Confirm] [<CommonParameters>]

 

Normally, when you use the ForEach-Object cmdlet, each object piped to the cmdlet is processed sequentially.

PS C:> 1..5 | ForEach-Object { "Hello $_"; sleep 1 } Hello 1 Hello 2 Hello 3 Hello 4 Hello 5 PS C:> (Measure-Command { 1..5 | ForEach-Object { "Hello $_"; sleep 1 } }).Seconds 5

But with the new ForEach-Object -Parallel parameter set, you can run all script in parallel for each piped input object.

PS C:> 1..5 | ForEach-Object -Parallel { "Hello $_"; sleep 1; } -ThrottleLimit 5
Hello 1
Hello 3
Hello 2
Hello 4
Hello 5

PS C:> (Measure-Command { 1..5 | ForEach-Object -Parallel { "Hello $_"; sleep 1; } -ThrottleLimit 5 }).Seconds
1

Because each script block in the ForEach-Object example above takes 1 second to run, running all five in parallel takes only one second instead of 5 seconds when run sequentially.

Since the script blocks are run in parallel for each of the 1-5 piped input integers, the order of execution is not guaranteed. The -ThrottleLimit parameter limits the number of script blocks running in parallel at a given time, and its default value is 5.

This new feature also supports jobs, where you can choose to have a job object returned instead of having results written to the console.

PS C:> $Job = 1..5 | ForEach-Object -Parallel { "Hello $_"; sleep 1; } -ThrottleLimit 5 -AsJob PS C:> $job | Wait-Job | Receive-Job Hello 1 Hello 2 Hello 3 Hello 5 Hello 4

ForEach-Object -Parallel is not the same as the foreach language keyword

Don’t confuse ForEach-Object cmdlet with PowerShell’s foreach keyword. The foreach keyword does not handle piped input but instead iterates over an enumerable object. There is currently no parallel support for the foreach keyword.

PS C:> foreach ($item in (1..5)) { "Hello $item" }
Hello 1
Hello 2
Hello 3
Hello 4
Hello 5

How does it work?

The new ForEach-Object -Parallel parameter set uses existing PowerShell APIs for running script blocks in parallel. These APIs have been around since PowerShell v2, but are cumbersome and difficult to use correctly. This new feature makes it much easier to run script blocks in parallel. But there is a fair amount of overhead involved and many times there is no gain in running scripts in parallel, and in fact it can end up being significantly slower than running ForEach-Object normally.

PowerShell currently supports parallelism in three main categories.

  1. PowerShell remoting. Here PowerShell sends script to external machines to run, using PowerShell’s remoting system.
  2. PowerShell jobs. This is the same as remoting except that script is run in separate processes on the local machine, rather than on external machines.
  3. PowerShell runspaces. Here script is run on the local machine within the same process but on separate threads.

This new feature uses the third method for running scripts in parallel. It has the least overhead of the other two methods and does not use the PowerShell remoting system. So it is generally much faster than the other two methods.

However, there is still quite a bit of overhead to run script blocks in parallel. Script blocks run in a context called a PowerShell runspace. The runspace context contains all of the defined variables, functions and loaded modules. So initializing a runspace for script to run in takes time and resources. When scripts are run in parallel they must be run within their own runspace. And each runspace must load whatever module is needed and have any variable be explicitly passed in from the calling script. The only variable that automatically appears in the parallel script block is the piped in object. Other variables are passed in using the $using: keyword.

$computers = 'computerA','computerB','computerC','computerD'
$logsToGet = 'LogA','LogB','LogC'

# Read specified logs on each machine, using custom module
$logs = $computers | ForEach-Object -ThrottleLimit 10 -Parallel {
    Import-Module MyLogsModule
    Get-Logs -ComputerName $_ -LogName $using:logsToGet
}

Given the overhead required to run scripts in parallel, the -ThrottleLimit becomes very useful to prevent the system from being overwhelmed. There are some cases where running a lot of script blocks in parallel makes sense, but also many cases where it does not.

When should it be used?

There are two primary reasons to run script blocks in parallel with the ForEach-Object -Parallel feature (keeping in mind that this feature runs the script on separate system threads).

  1. Highly compute intensive script. If your script is crunching a lot of data over a significant period of time and the scripts can be run independently, then it is worthwhile to run them in parallel. But only if the machine you are running on has multiple cores that can host the script block threads. In this case the -ThrottleLimit parameter should be set approximately to the number of available cores. If you are running on a VM with a single core, then it makes little sense to run high compute script blocks in parallel since the system must serialize them anyway to run on the single core.
  2. Script that must wait on something. If you have script that can run independently and performs long running work that requires waiting for somethings to complete, then it makes sense to run these tasks in parallel. If you have 5 scripts that take 5 minutes each to run but spend most of the time waiting, you can have them all run/wait at the same time, and complete all 5 tasks in 5 minutes instead of 25 minutes. Scripts that do a lot of file operations, or perform operations on external machines can benefit by running in parallel. Since the running script cannot use all of the machine cores, it makes sense to set the -ThrottleLimit parameter to something greater than the number of cores. If one script execution waits many minutes to complete, you may want to allow tens or hundreds of scripts to run in parallel.
$logNames.count
10

PS C:> Measure-Command { $logs = $logNames | ForEach-Object -Parallel { Get-WinEvent -LogName $_ -MaxEvents 5000 2>$null } -ThrottleLimit 10 }
TotalMilliseconds : 115994.3 (1 minute 56 seconds)
$logs.Count
50000

PS C:> Measure-Command { $logs = $logNames | ForEach-Object { Get-WinEvent -LogName $_ -MaxEvents 5000 2>$null } }
TotalMilliseconds : 229768.2364 (3 minutes 50 seconds)
$logs.Count
50000

The script above collects 50,000 log entries on the local machine from 10 system log names. Running this in parallel is almost twice as fast as running sequentially, because it involves some relatively slow disk access and can also take advantage of the machine multiple cores as it processes the log entries.

When should it be avoided?

ForEach-Object -Parallel should not be thought as something that will always speed up script execution. And in fact it can significantly slow down script execution if used heedlessly. For example, if your script block is executing trivial script then running in parallel adds a huge amount of overhead and will run much slower.

PS C:> (measure-command { 1..1000 | ForEach-Object -Parallel { "Hello: $_" } }).TotalMilliseconds
10457.962

PS C:> (measure-command { 1..1000 | ForEach-Object { "Hello: $_" } }).TotalMilliseconds
18.4473

The above example, a trivial script block is run 1000 times. The ThrottleLimit is 5 by default so only 5 runspace/threads are created at a time, but still a runspace and thread is created 1000 times to do a simple string evaluation. Consequently, it takes over 10 seconds to complete. But removing the -Parallel parameter and running the ForEach-Object cmdlet normally, results in completion in about 18 milliseconds.

So, it is important to use this feature wisely.

Implementation details

As previously mentioned, the new ForEach-Object -Parallel feature uses existing PowerShell functionality to run script blocks concurrently. The primary addition is the ability to limit the number of concurrent scripts running at a given time with the -ThrottleLimit parameter. Throttling is accomplished by a PSTaskPool class that holds running tasks (running scripts), and has a settable size limit which is set to the throttle limit value. An Add method allows tasks to be added to the pool, but if it is full then the method blocks until a new slot becomes available. Adding tasks to the task pool was initially performed on the ForEach-Object cmdlet piped input processing thread. But that turned out to be a performance bottleneck, and now a dedicated thread is used to add tasks to the pool.

PowerShell itself imposes conditions on how scripts run concurrently, based on its design and history. Scripts have to run in runspace contexts and only one script thread can run at a time within a runspace. So in order to run multiple scripts simultaneously multiple runspaces must be created. The current implementation of ForEach-Object -Parallel creates a new runspace for each script block execution instance. It may be possible to optimize this by re-using runspaces from a pool, but one concern in doing this is leaking state from one script execution to another.

Runspace contexts are an isolation unit for running scripts, and generally do not allow sharing state between themselves. However, variables can be passed at the beginning of script execution through the $using: keyword, from the calling script to the parallel script block. This was borrowed from the remoting layer which uses the keyword for the same purpose but over a remote connection. But there is a big difference when using the $using: keyword in ForEach-Object -Parallel. And that is for remoting, the variable being passed is a copy sent over the remoting connection. But with ForEach-Object -Parallel, the actual object reference is being passed from one script to another, violating normal isolation restrictions. So it is possible to have a non thread-safe variable used in two scripts running on different threads, which can lead to unpredictable behavior.

# This does not throw an error, but is not guaranteed to work since the dictionary object is not thread safe
$threadUnSafeDictionary = [System.Collections.Generic.Dictionary[string,object]]::new()
Get-Process | ForEach-Object -Parallel {
    $dict = $using:threadUnSafeDictionary
    $dict.TryAdd($_.ProcessName, $_)
}
# This *is* guaranteed to work because the passed in concurrent dictionary object is thread safe
$threadSafeDictionary = [System.Collections.Concurrent.ConcurrentDictionary[string,object]]::new()
Get-Process | ForEach-Object -Parallel {
    $dict = $using:threadSafeDictionary
    $dict.TryAdd($_.ProcessName, $_)
}

$threadSafeDictionary["pwsh"]

 NPM(K)    PM(M)      WS(M)     CPU(s)      Id  SI ProcessName
 ------    -----      -----     ------      --  -- -----------
    112   108.25     124.43      69.75   16272   1 pwsh

Conclusion

This feature can greatly improve your life for many work load scenarios. As long as you understand how it works and what its limitations are, you can experiment with parallelism and make real performance improvements with your scripts.

Paul Higinbotham
Senior Software Engineer
PowerShell Team

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