Author: Angelo Rubini

Original post on Foojay: Read More

Profiling a Java Application Running Inside an Unikernel with JProfiler

Unikernels are often associated with minimalism and tight resource control.

But can we profile a Java application running inside a unikernel using a standard JVM profiler?

The answer is yes.

In this guide, we will walk step by step through profiling a Quarkus
application running inside a Nanos unikernel using JProfiler and
IBM Semeru JRE 25 (OpenJ9).

No special hacks. Just standard JVM tooling.

---

Prerequisites

• Ops (https://ops.city/)
• JProfiler (https://www.ej-technologies.com/jprofiler)
• A Quarkus application packaged as a runnable JAR

---

1. Project Structure

We start from a clean Proof of Concept directory:

quarkusProfiling$ ls
config.json  jprofiler15  myQuarkusApp  tmp

Directory breakdown:

• config.json → Ops/Nanos configuration file
• jprofiler15/ → extracted JProfiler distribution
• myQuarkusApp/ → packaged Quarkus runnable JAR
• tmp/ → writable directory inside the unikernel

---

2. Ops Nanos Configuration

Below is the complete config.json used for this setup:

{
    "Version": "1.0.0",
    "Dirs":["myQuarkusApp","jprofiler15","tmp"], 
    "Args": [
        "-agentpath:/jprofiler15/bin/linux-x64/libjprofilerti.so=port=8849,address=0.0.0.0",
        "-jar",
        "myQuarkusApp/quarkus-faces-runner.jar"
    ],
    "RunConfig": {
        "Ports": ["8080","8849"]
    },
    "BaseVolumeSz": "300m"  
}

Explanation

Dirs

"Dirs":["myQuarkusApp","jprofiler15","tmp"]

These directories are embedded into the unikernel filesystem:

• The application
• The JProfiler native agent
• A writable tmp directory

---

Args

"-agentpath:/jprofiler15/bin/linux-x64/libjprofilerti.so=port=8849,address=0.0.0.0"

This is the key configuration.

We explicitly load the JProfiler native JVMTI agent at JVM startup.

Important parameters:

• port=8849 → port used by the profiler
• address=0.0.0.0 → listen on all interfaces

Then we launch the application:

"-jar", "myQuarkusApp/quarkus-faces-runner.jar"

---

RunConfig

"Ports": ["8080","8849"]

We expose:

• 8080 → Quarkus HTTP endpoint
• 8849 → JProfiler agent port

---

BaseVolumeSz

"BaseVolumeSz": "300m"

Allocates 300 MB for the unikernel filesystem.

---

3. Starting the Unikernel

From the project root directory, run:

ops pkg load AngeloRubens/SemeruJREx64Linux:25.0.1 --nightly -c config.json

What this command does

• Loads IBM Semeru JRE 25 (OpenJ9)
• Applies our local config.json
• Embeds the application and profiler agent
• Boots the unikernel locally

There is no separate container or VM runtime.
The JVM runs directly inside the unikernel.

---

4. Boot Log Analysis

When the unikernel starts, you will see:

running local instance
booting /home/lop/.ops/images/java ...
[0.227374] en1: assigned 10.0.2.15

The unikernel has booted and received an IP address.

Then the JVM starts and the profiler agent initializes:

JProfiler> Protocol version 68
JProfiler> OpenJ9 JVMTI detected.
JProfiler> Java 25 detected.
JProfiler> 64-bit library
JProfiler> Listening on port: 8849.

This confirms:

• OpenJ9 JVM detected
• Java 25 runtime
• JVMTI available
• Native agent loaded successfully

---

Instrumentation Phase

JProfiler> Enabling native methods instrumentation.
JProfiler> Can retransform classes.
JProfiler> Native library initialized
JProfiler> VM initialized

This means:

• Class retransformation is supported
• Native methods can be instrumented
• The agent is fully integrated

---

Final State

JProfiler> Retransforming 11 base class files.
JProfiler> Base classes instrumented.
JProfiler> Waiting for a connection from the JProfiler GUI ...
JProfiler> Using address 0.0.0.0.

At this point:

• The JProfiler agent is listening on port 8849
• The JVM is waiting for a GUI connection

The profiling infrastructure inside the unikernel is fully operational.

---

5. Connecting JProfiler GUI

Open JProfiler.

1. Click New Session
2. Select Attach to remote JVM
3. Enter:
   • Host: 127.0.0.1
   • Port: 8849
4. Connect

The GUI will attach to remote JVM running inside the unikernel(or connect to remote jvm running in you cloud provider or onprem).

From here you can:

• Inspect CPU hotspots
• Analyze memory allocations
• Capture heap snapshots
• Monitor threads
• View GC activity

Exactly as you would with a standard JVM.

SO Info(Nanos name):

CPU Hot Spots

CPU Call Tree

Live Objects

---

Why This Works

Unikernels do not remove JVM capabilities.

JProfiler uses:

• JVMTI
• Native agent loading via -agentpath
• Standard TCP communication

As long as:

• The agent library is available
• The port is exposed
• The JVM supports JVMTI

Profiling works.

The environment (VM, container, unikernel) does not change the JVM instrumentation model.

---

Conclusion

Profiling a Java application inside a unikernel is straightforward.

By:

• Embedding the profiler native agent
• Exposing the profiler port
• Using a standard JVM (IBM Semeru OpenJ9)

You can use JProfiler — or any JVMTI-based tool — without
limitations.

Unikernels do not prevent observability.
They simply run your JVM in a different execution model.

And from the JVM perspective, everything works exactly as expected.

If you want, yoou can see the sample app of this post, running into nanos Unikernel on Arm64 Ampere Oracle OCI Cloud http://80.225.94.232:8080/index.xhtml

The post Unikernel: Profiling and Troubleshooting JVM on Nanos Unikernel appeared first on foojay.