Application Insights: "Unmanaged async" in profiler trace

Question

We have high CPU load on one of our web-app instances and I'm trying to investigate the cause. There are some profile traces recorded in AI. All of them contain a very long "Unmanaged Async" event.

What does this "Unmanaged Async" mean?

Answer 1

.Net framework emits ETW events and passes activity ids between threads so that async calls can be tracked across threads. Unmanaged code (native code) and some older styles of asynchronous code are missing these events and activity ids, so the profiler cannot track which thread is running code and what code is running. This is labeled 'unmanaged async' in the call stack. If you download the ETW file, you may be able to use perfview to get more insight into what is happening.