Debugging Multithreaded Applications
Multithreaded applications are always harder to debug as you have to track multiple threads at a time. Moreover, multithreaded applications introduce new types of bugs and performance issues like uneven workload distribution, lock contention, serialized execution, and other*.
The illusion of a sequential flow you get from stepping in a debug session does not help while debugging a multithreaded app and can be misleading. When investigating issues that can be concurrency bugs, you should rely more on breakpoints rather than on sequential stepping. For example, conditional breakpoints can help you a lot if you want to focus on just one thread: simply use some unique property of your thread (e.g. thread ID) to trigger the breakpoint.
JetBrains Rider provides the following functionality to simplify the debugging of multithreaded applications:
- Frames pane allows you to select a particular thread and view its stack frame.
- Threads pane shows all threads and their stack frames as a tree.
- Parallel Stacks pane simplifies threads analysis by showing threads and their stack frames on the diagram. See details below in this section.
For example, let's take a look at how Parallel Stacks look like during the debugging of a simple application that starts a number of threads. Each thread does nothing but sleeps for one second via
Thread.Sleep(1000). Let's imagine, we've put a breakpoint at this
Thread.Sleep(1000) call and stopped on fourth breakpoint hit: four threads were created up to that moment.
Here are some notes to the image: