The CLR ThreadPool has two types of threads - "Worker" and "I/O Completion Port" (aka IOCP) threads.
- Worker threads are used when for things like processing
Task.Run(…)orThreadPool.QueueUserWorkItem(…)methods. These threads are also used by various components in the CLR when work needs to happen on a background thread. - IOCP threads are used when asynchronous IO happens (e.g. reading from the network).
IOCP threads are most interesting from a StackExchange.Redis viewpoint, so we will focus on those.
The thread pool provides new worker threads or I/O completion threads on demand until it reaches the "Minimum" setting for each type of thread. By default, the minimum number of threads is set to the number of processors on a system.
Once the number of existing (busy) threads hits the "minimum" number of threads, the ThreadPool will throttle the rate at which is injects new threads to one thread per 500 milliseconds. This means that if your system gets a burst of work needing an IOCP thread, it will process that work very quickly. However, if the burst of work is more than the configured "Minimum" setting, there will be some delay in processing some of the work as the ThreadPool waits for one of two things to happen 1. An existing thread becomes free to process the work 2. No existing thread becomes free for 500ms, so a new thread is created.
Basically, it means that when the number of Busy threads is greater than Min threads, you are likely paying a 500ms delay before network traffic is processed by the application.
If we look at an example error message from StackExchange.Redis (build 1.0.450 or later), you will see that it now prints ThreadPool statistics (see IOCP and WORKER details below).
System.TimeoutException: Timeout performing GET MyKey, inst: 2, mgr: Inactive,
queue: 6, qu: 0, qs: 6, qc: 0, wr: 0, wq: 0, in: 0, ar: 0,
IOCP: (Busy=6,Free=994,Min=4,Max=1000),
WORKER: (Busy=3,Free=997,Min=4,Max=1000)
In the above example, you can see that for IOCP thread there are 6 busy threads and the system is configured to allow 4 minimum threads. In this case, the client would have likely seen two 500 ms delays because 6 > 4.
Given the above information, we strongly recommend that customers set the minimum configuration value for IOCP threads to something larger than the default value. We can't give one-size-fits-all guidance on what this value should be because the right value for one application will be too high/low for another application. This setting can also impact the performance of other parts of complicated applications, so each customer needs to fine-tune this setting to their specific needs. A good starting place is 200 or 300, then test and tweak as needed.
How to configure this setting:
- In ASP.NET, use the "minIoThreads" configuration setting under the
<processModel>configuration element. - Outside of ASP.Net, use the ThreadPool.SetMinThreads(…) API.