Tips and tricks

  1. Collect enough samples
  2. Trigger JIT compiler before profiling
  3. Supply custom agent binary

Collect enough samples

The profiling library under the hood of clj-async-profiler, async-profiler, is a sampling profiler. It means that it is not 100% accurate; instead, it derives the profile picture statistically. When it comes to statistics, you need enough data samples to be confident about the results. So, make sure that when you profile anything with clj-async-profiler, you get at least 1000 samples. The empirically optimal number of samples for :cpu event type is 5000-10000. This roughly corresponds to 5-10 seconds of running time when the CPU is fully loaded; so when you want to measure something that finishes faster, then repeat it in a loop that runs at least for a few seconds.

;; 1000 iterations is an example. You'll have to tune the number for your
;; particular code to achieve the 5-10 seconds runtime.

user=> (prof/profile
        (dotimes [_ 1000]

When profiling other events (e.g., :alloc, :lock), you might not be able to get that many samples, but that is alright.

You can learn how many samples the profiler collected by hovering over the bottom stackframe in the flamegraph. The number of samples is also displayed on the UI page next to the flamegraph name.

Trigger JIT compiler before profiling

When you profile some piece of code, it is a good idea to run it on its own once or even twice without profiling. If the form is too quick to execute and you wrap it in dotimes, run the whole dotimes before profiling. This ensures that the underlying code is compiled to native instructions by the JIT compiler. Otherwise, you might be partially profiling the interpreted version of the code that may have a completely different performance behavior than the final C2-compiled version.

Supply custom agent binary

clj-async-profiler ships with the following agent binaries. To use clj-async-profiler on other supported platforms, you should do the following:

  1. Build async-profiler for the desired platform.
  2. Put the resulting in a place accessible by your JVM process (and which also allows code execution from).
  3. Execute from Clojure:
    user=> (reset! prof/async-profiler-agent-path "/path/to/")