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Node Profiling with V8

In this lab, you will experiment with profiling a CPU bottleneck in a Node.js application using the built-in V8 profiler. The key advantage of using the built-in profiler is that it is, well, built-in: you don't need any additional software, and it works across all platforms. On the other hand, it requires that you restart your Node application with a special profiling switch, which you can't leave on for a long duration of time.

Task 1: Run the Application with a Benchmark

Navigate to the nodey directory. If you haven't yet, you should make sure all required packages are installed by running npm install. Then, run the following command to start our simple Node application:

$ ./run.sh

This starts the web server asynchronously. One of its endpoints is a simple authentication service, which takes a username and password and returns user information. You can try it with curl:

$ curl -X POST 'http://localhost:3000/users/auth?username=foo&password=wrong'

Let's ignore for a second the horrible practice of passing the username and password as part of the query string, and focus on the performance of this endpoint. Run the following command to get a simple local benchmark of its behavior:

$ ab -c 10 -n 100 -m POST 'http://localhost:3000/users/auth?username=foo&password=wrong'

In another shell, you can run top to get a quick idea of the system's behavior. You can clearly see the Node process consuming lots of CPU. The benchmark results are not very optimistic, either; the mean time to service an authentication request is quite high.

Task 2: Run the Application with the V8 Profiler

To profile the application, we will use the built-in V8 profiler. The wrapper script, run.sh, can invoke the Node runtime with the --prof switch, which turns on the profiler. The profiler output goes to a file named isolate-nnnnn.log, which can then be parsed and analyzed.

Run the application with the profiler enabled:

$ ./run.sh prof

Run the benchmark again:

$ ab -c 10 -n 100 -m POST 'http://localhost:3000/users/auth?username=foo&password=wrong'

Now, look in the current directory for the isolate-nnnnn.log file. You don't have to parse it by hand; you can ask the Node runtime to parse it and produce a text report:

$ node --prof-process isolate*.log | less

Inspect the report and try to answer the following questions:

  • Which JavaScript method was responsible for most of the CPU usage?
  • What's the proportion of time spent in JavaScript code, C++ code, or GC code?
  • Which call stack leads to the most CPU-consuming function?