Scripts have to run in runspace contexts and only one script thread can run at a time within a runspace. PowerShell itself imposes conditions on how scripts run concurrently, based on its design and history. But that turned out to be a performance bottleneck, and now a dedicated thread is used to add tasks to the pool. Adding tasks to the task pool was initially performed on the ForEach-Object cmdlet piped input processing thread. 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. 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. The primary addition is the ability to limit the number of concurrent scripts running at a given time with the -ThrottleLimit parameter. Implementation detailsĪs previously mentioned, the new ForEach-Object -Parallel feature uses existing PowerShell functionality to run script blocks concurrently. So, it is important to use this feature wisely. But removing the -Parallel parameter and running the ForEach-Object cmdlet normally, results in completion in about 18 milliseconds. Consequently, it takes over 10 seconds to complete. 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. The above example, a trivial script block is run 1000 times.
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