Report of my trip to JavaZone and northern Germany

Between 2nd and 17th September, I gave three talks in three different cities:

I traveled from Karlsruhe to Oslo (via Stuttgart Airport) and from Oslo to Hanover via plane, then to Hamburg, Hanover, back to Hamburg, and the end via Bonn back to Karlsruhe via train:

This was my first time traveling to a conference by plane because traveling to Oslo by train takes far longer (20 hours or more).

This was my second two-week-long tour giving talks, after my tour d’Europe in May/June this year (see Report of my small Tour d’Europe), but this time it consisted solely of talks at conferences. The following is a short report of my trip that saw me brewing beer, giving a talk at one of my favorite conferences, and visiting Hamburg for the first time.


I started by traveling to Oslo on Saturday before the conference, which began on Tuesday with workshops. My journey started with the 2:30 am bus from Karlsruhe to Stuttgart, where I took the 6:30 am flight to Paris and from there to Oslo. Flying in the early hours of the morning is something extraordinary:

I’ve never really been to Oslo before, except as a toddler, according to my parents, so I wanted to explore the city. I arrived in Oslo Saturday afternoon and stayed till Tuesday morning at the home of a friendly expat that I met via the couch-surfing platform, staying there till Tuesday morning. It was great: I had the opportunity to visit the famous Fram Polar Exploration Museum, hike around the Vettakollen, and brew beer with my host:


Then, on Tuesday, it was time for the workshop day of JavaZone. I’ve been drawn to this conference since I got introduced to their conference trailers in my first semester at university:

So, speaking, there was a great honor; I can recommend this experience to anyone. It is a lovely venue with good food and great organizers who care about their speakers (shout out to Felix Rabe, Rafael Winterhalter, and Marek Machnik).

I started the conference by attending the “A little taste of testing the Java compiler” workshop by Hasnae Rehioui:

Hasnae Rehioui, in her workshop

If you want to start with building the OpenJDK and running JTREG tests, her accompanying website is an excellent place to start.

At the end of the day, I went to the speaker dinner, where I met the organizers and people like Fabian Stäber and Gunnar Morling. The view from the restaurant was majestic:

Then, the next day, the conference began. The conference paid for the hotel during the week. I met Pasha Finkelshtein and Marit van Dijk from JetBrains at the breakfast buffet. I was that day in many talks, including the Maven Puzzlers talk by Andres Almiray and Ixchel Ruiz:

The day ended with eating with a few of my fellow speakers and going to the AweZone party at Himkok bar, but only after a musical performance by Mr. Orkester:

What a great way to inform all JavaZone participants of the looming AweZone event.

The next day was full of talks, including mine and eating great food. I now understand why some people call JavaZone affectionately FoodZone, with all the food served all day. My talk was in the first slot at 9:00 am; I was amazed that around 150 people turned up to attend it after the long night at the bar:

Thank you to Inna Belyantseva for the great Math-as-a-Service logo and valuable feedback on my presentation style in the weeks before the conference.

Later in the day, I went to Theresa Mammarella’s talk on CVEs:

I was introduced to her the evening before, and it was great to see the only other young JDK developer (in her case, OpenJ9) on stage.

In the evening, I ate dinner with a couple of my fellow speakers, including Marit van Dijk, Rustam Mehmandarov, Gerrit Grunwald, Raquel Pau, Tim te Beek, Steve Poole, Alina Yurenko, Theresa Mammarella, Mads Opheim and Ko Turk. Afterward, we went to Himkok bar to relax after two days at the conference:

The next day was my last in Oslo before moving to northern Germany. So I took the chance to explore the city center once more and made a photograph of the speaker’s gift:

A Viking duke in front of the Oslo Stock Exchange

I had to say goodbye to Oslo and then traveled to Hamburg via Hanover Airport to stay with a friend in Hamburg.


The problem was that JavaZone pays the hotel till Friday, and the speaker dinner for Java Forum Nord is Monday evening. I knew someone in Hamburg, so I stayed in Hamburg from Friday night till Monday afternoon. I used the time to look into Python debugging, which eventually resulted in my Let’s create a Python Debugger together: Part 1 blog post, went sightseeing, watched the dark comedy Sophia, der Tod und Ich, and visited the Hamburger Miniaturwunderland:

While there, I had the pleasure of traveling with light luggage, as my luggage didn’t arrive in Hamburg till Wednesday because it was somehow stuck in Amsterdam airport, where I had a stopover.

Java Forum Nord

The second conference on my journey was the Java Forum Nord in Hanover on Tuesday. This conference is a small community-run event, without many sponsors and many talks in German. It was my first German Java conference, so there were many German-only speakers that I hadn’t seen at a conference before. I met a few of them at the speaker’s dinner on Monday and at the after-party on Tuesday:

Speakers’ dinner with alcohol-free beer and Spätzle in Hanover

I enjoyed meeting Marit van Dijk again and getting to know Sandra Parsick and Karl Heinz Marbaise.

The day after, I traveled back to Hamburg for the code.talks conference.


This was the third conference in a row and the only one without a focus on Java. The talks were on various technologies, from NFTs to creativity. The most memorable of the conference was by far the speaker’s dinner at the open kitchen restaurant Hensslers Küche and visiting the Heavens Bar & Kitchen rooftop bar in St. Pauli with Jacqueline Franßen, Hannes Drittler, and Samir Ar after the second day of the conference:

While there, I also started preparing an upcoming talk for JCon World with Marit van Dijk and had an online meeting with Jaroslav Bachorik and Erik Österlund on the future of my JEP, so stay tuned.

I traveled back to Karlsruhe via Bonn, where I met a good friend and attended a Haydn concert in the Kreuzkirche, closing the eventful two weeks cooking flammkuchen together with her and one of her new flatmates.


Giving three talks at conferences in a row was an experience I’m grateful for. Talking with many new and old acquaintances was, at times, taxing, yet definitely worthwhile. Especially JavaZone was a conference I never dreamt of being able to speak at.

I’m looking forward to giving many more in the future and happy that the SapMachine team at SAP allows me to do so and fully supports me in my endeavors.

If you want to attend one of my talks, just come to Devoxx Belgium, BaselOne, or J-Fall, where I’ll speak this autumn. You can find all the meet-ups and conferences that I’ve confirmed on my Talks page. I’m happy to talk at your meet-up or conference; just ask me.

This project is part of my work in the SapMachine team at SAP, making profiling easier for everyone.

C2 might slow down your builds

At the JavaForumNord two weeks ago, I had a friendly chat with Karl Heinz Marbaise (Chairman of the Apache Maven Project), where he mentioned that he wanted to start profiling maven. This sounded interesting, so I started looking into the performance and bottlenecks of maven. I began by using the Maven build of maven itself as a starting point (excluding the tests). The following is my first observation related to maven builds in CIs.

Maven is one of the most used build systems in the Java ecosystem (JetBrains 2022 survey), and many projects, especially open-source projects, use GitHub Actions to create artifacts and run tests automatically. The free tier of GitHub Actions has two cores (see GitHub docs), so we only have limited parallelism compared to the usual developer machines.

Maven uses the Plexus compiler wrapper Java API around the API to compile every Java file. The underlying JVM then essentially has three tasks running while compiling:

  • Maven main thread that does the compilation
  • Garbage Collection thread for memory management
  • JIT compilation thread to compile the maven byte code (it’s getting meta), enabling it to run faster (see Mastering the Art of Controlling the JIT: Unlocking Reproducible Profiler Tests for more information). Especially the C2 compiler takes some time. This is our example in a shared thread with the GC.

Small builds

This is problematic on short builds (like building maven itself in 30s), as the C2 JIT does cost a significant amount of cpu-time, more than is saved by the faster execution of the jitted code. The maven self-built, for example, spent more than half of its cpu-time in the C2 compiler:

Maven 4 (f24266eb64) was built with maven 3.8.7 on SapMachine and profiled with async-profiler; click to view the full flame graph

We can use the information by async-profiler to get the cpu-time proportions for significant parts of the built:

Partcpu-time percentagesamples
Launcher.main38 %3200
C2Compiler::compile_method47 %4000
Compiler::compile_method (C1 compiler)8 %650

The whole maven build took 44s (82s cpu-time) on two cores of my ThreadRipper 3995WX. Compare this to a run with a disabled C2 which took 41s (50s cpu-time):

Maven 4 (f24266eb64) built with maven 3.8.7 on SapMachine and MVN_OPTS="-XX:TieredStopAtLevel=1" (but stopping at a higher C1 level is unusual, see dzone) profiled with async-profiler, click to view the full flame graph

The proportions are as expected:

Partcpu-time percentagesamples
Launcher.main71 %3700
C2Compiler::compile_method0 %0
Compiler::compile_method (C1 compiler)14 %750

The time to complete the task changes only by 3s, but the CPU time and, by proxy, the energy use drops significantly. These results are stable over multiple runs with different JDKs.

Side note: Maven only compiles with one thread, compiling with two threads (-T2 option) reduces the build time further by 10 to 15% with C2 and without.

Does this mean that you should always disable C2 in CI builds? No. At a certain project size, the performance increase by the faster, compiled methods outweighs the C2 compilation costs.

Large builds

Take, for example, quarkus: A clean build on two cores runs for 430s (710s cpu-time) with C2 enabled:

Quarkus (af4208a05e) built with maven 3.8.8 on SapMachine and ./mvnd -Dquickly

The runtime proportions are less skewed in the direction of C2:

Partcpu-time percentagesamples
MavenWrapperMain.main45 %39200
C2Compiler::compile_method30 %26200
Compiler::compile_method (C1 compiler)2 %1750

The built without C2 runs in comparison for 880s (1000s cpu-time), so it doesn’t make any sense to disable C2 with this build. For completeness, the proportion table:

Partcpu-time percentagesamples
MavenWrapperMain.main57 %52700
C2Compiler::compile_method0 %0
Compiler::compile_method (C1 compiler)7 %6150


Disabling C2 can be an option to speed up builds of smaller Java applications in CI systems, mainly when restricted to one or two CPU cores. I would recommend exploring this for every maven build that runs under a minute, as it is not too hard to integrate (MVN_OPTS="-XX:TieredStopAtLevel=1"), yet might result in less CPU usage. Preliminary findings also show that it reduces memory usage. But my findings also show that it is not helpful for large builds.

I hope you enjoyed this explorative blog post in the realm of JIT compilation. It’s preliminary research; maybe if there’s enough interest, I can do a more thorough investigation. See you in my next blog post on debugging, which should be ready by the end of this week.

Additional Literature: I would recommend reading Startup, containers & Tiered Compilation by Jean-Philippe Bempel if you want to get another take on the impact of C2 on startup time in constrained environments.

This project is part of my work in the SapMachine team at SAP, making profiling easier for everyone. Thank you to Francesco Nigro for the idea to check whether disabling the JIT improves the build times.