Node.js Performance Monitoring – Part 3: Debugging the Event Loop


Take a deep dive into the Node.js event loop and learn how to diagnose and debug issues that stem from unoptimized JavaScript.

In Part 1 and Part 2 of this series on Node.js Performance Monitoring, we covered the basic metrics to monitor and how to do this with N|Solid.
In this article, we’re going to dig a bit deeper into how to diagnose and resolve common Node.js bottlenecks in the event loop by understanding some of the key metrics around the event loop.
Understanding Parts of the Event Loop
A “task” is a unit of work to perform after completing an asynchronous operation. This should not be confused with event emitted on the EventEmitter; which only counts for a portion of where time is spent in task execution. For example, processing an http request also includes parsing the headers; which is done before the ‘request’ event is called.
The event loop cannot process multiple tasks simultaneously. Instead tasks are placed in a queue and processed sequentially. This means one one task can be held up because the one before it is taking a long time to complete. A bottleneck can be identified by observing an abnormally large amount of lag between processing asynchronous tasks. Because all task execution originates from the event loop it is the best location for initial bottleneck measurements.
An event loop without lag (we’ll consider “healthy”) will show that no task has been queued to wait for the completion of another task. Instead they are all processed the moment they are received.
In Node.js it is impossible to determine the moment a task is received. Only when it is ready to be processed. This is because Node.js relies on the operating system’s kernel to handle the task queue. Node.js itself has no insight into this. Thus if 2 tasks are ready to be processed at the same time Node can only know the second task is available after the first has been completed. So, for example, if your application requires low latency responding to TCP requests it is important to know that no two tasks have overlapped.

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