Understanding Sitecore's Self-Adjusting Thread Pool Size Monitor

Background

I must admit, I have become mildly obsessed with the threading over the last couple years, mostly because a great deal of my work has involved stabilization and optimization practices on high-traffic Sitecore sites. 

In this post, I want to focus in the Thread Pool Size Monitor that comes baked into Sitecore from 9 onwards, because it is not widely known that it exists, the job it does, and how it can be tuned to optimize performance.

Thread Pool Size Monitor

To recap, the most important thread configuration settings are the minWorkerThreads and minIOThreads where you can specify the minimum number of threads that are available to your application's Thread Pool instead of relying on the default formula's based on processor count which is always too few.

Threads that are controlled by these values can be created at a much faster rate (because they are spawned from the Thread Pool), than worker threads that are created from the CLR's default "thread-tuning" capabilities. 

To summarize: 

• Thread pool threads get thrown in faster to handle work. 
• The CLR thread spin up algorithm is too slow and we can't rely on it to support high performance applications.

As previously mentioned, in Sitecore 9 and above, there is a pipeline processor that allows the application to adjust thread limits dynamically based on real-time thread availability (using the Thread Pool API).

This can be found in the following namespace: Sitecore.Analytics.ThreadPoolSizeMonitor.

By default, every 500 milliseconds, the processor will keep adding a value of 50 to the minWorkerThreads setting via the Thread Pool API until it determines that the minimum number of threads is adequate based on available threads.

How It Works

Since a picture is worth a thousand words, I put together a diagram of how the logic of the Thread Pool Size Monitor logic works, and provided an example with the default settings that are set on an Azure P3v2 App Service that has 4 cores.  

Custom Thread Pool Size Monitor Configuration

An enhancement that I have made on my past PaaS implementation was to tune Sitecore’s dynamic thread processor using a more “aggressive” configuration. This helped me with those “bursty” web traffic situations where I needed to be sure that I had enough threads available to serve the current demands. 

Here is the configuration that I used: