Extracting cpu profile and memory dump from a live Express.js app

Still in the same vibe of How I solved a memory leak in a NodeJS worker, I’m going to describe how I created an entrypoint on Express.js to extract cpu profile logs and heap dump using v8-profiler.

The first problem that I’ve found was a segfault when trying to generate the heap dumps, that I promptly noticed in the issue tracker that it is a compatibility bug related to node 8.x. Following a user’s suggestion, I replaced the v8-profile by v8-profile-node8 for now. Despite the horrible documentation and random v8-profiler is pretty handy.

Here is my code. Enjoy:

import * as profiler from "v8-profiler-node8";
if (process.env.ENABLE_PROFILE) {
  router.get("/admin/memory_profile", (_req, res) => {
    var snapshot = profiler.takeSnapshot();
    let contentDisposition = `attachment; filename="${process.pid}-${Date.now()}.heapsnapshot"`;
    res.setHeader("Content-type", "application/pdf");
    res.setHeader("Content-disposition", contentDisposition);
    snapshot
      .export()
      .pipe(res)
      .on("finish", () => {
        console.log("finished sending dump");
        snapshot.delete();
      });
  });

  // Only allow one capture per thread
  let capturingCpuProfile = false;
  router.get("/admin/cpu_profile", (_req, res) => {
    if (capturingCpuProfile) {
      return res.status(400).send("CPU profile being captured");
    }
    capturingCpuProfile = true;

    profiler.startProfiling();

    // Let it capture the cpu log for 30 seconds and then downloads the file
    setTimeout(() => {
      capturingCpuProfile = false;
      const profile = profiler.stopProfiling();

      let contentDisposition = `attachment; filename="${process.pid}-${Date.now()}.cpuprofile"`;
      res.setHeader("Content-type", "application/pdf");
      res.setHeader("Content-disposition", contentDisposition);

      profile
        .export()
        .pipe(res)
        .on("finish", function () {
          profile.delete();
        });
    }, 30000);
  });
}

I used that ENABLE_PROFILE environment variable because I don’t want to let it available on production as default. It’s a good idea to prevent unauthorized access too.

After downloading the files, just load them on chrome dev tools. For memory leak debugging, take a look at my previous post.

Caveats

Generating a memory dump uses a lot of memory, so make sure that the process is using less than half of the available memory.
The memory dump file must have the extension .heapdump
The cpu profile log must have the extension .cpuprofile

Memory consumption