Monthly Archives: September 2024

Mapping Java Thread Ids to OS Thread Ids

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This week, a short blog post on a question that bothered me this week: How can I get the operating systems thread ID for a given Java thread? This is useful when you want to deal with Java threads using native code (foreshadowing another blog post). The question was asked countless times on the internet, but I couldn’t find a comprehensive collection, so here’s my take. But first some background:

Background

In Java, normal threads are mapped 1:1 to operating system threads. This is not the case for virtual threads because they are multiplexed on fewer carrier threads than virtual threads, but we ignore these threads for simplicity here.

But what is an operating system thread? An operating system thread is an operating system task that shares the address space (and more) with other thread tasks of the same process/thread group. The main thread is the thread group leader; its operating system ID is the same as the process ID.

Be aware that the Java thread ID is not related to the operating system ID but rather to the Java thread creation order. Now, what different options do we have to translate between the two?

Different Options

During my research, I found three different mechanisms:

  1. Using the gettid() method
  2. Using JFR
  3. Parsing thread dumps

In the end, I found that option 3 is best; you’ll see why in the following.

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Hello eBPF: Writing a Linux scheduler in Java with eBPF (15)

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Or: Learn how to write a performant* Linux scheduler in 25 lines of Java code.

Welcome back to my series on ebpf. In the last post, I presented a recording of my JavaZone presentation on eBPF and a list of helpful resources for learning about the topic. Today, I’ll show you how to write a Linux scheduler in Java with eBPF. This blog post is the accompanying post to my eBPF summit keynote of the same title:

With my newest hello-ebpf addition, you can create a Linux scheduler by just implementing the methods of the Scheduler interface, allowing you to write a small scheduler with ease:

Is it really as easy as that? Of course not, at least not yet. Developing and running this scheduler requires a slightly modified version of hello-ebpf, which lives in the branch scx_demo, and a kernel patched with the sched-ext extension or a CachyOS instance with a 6.10 kernel, as well as some luck because it’s still slightly brittle.

Nonetheless, when you get it working, you can enter the wondrous world of people who build their schedulers with eBPF. You can find some of them on the sched-ext slack and many of their creation in the sched-ext/scx repository on GitHub. The kernel patches will hopefully be merged into the mainline kernel soon and will be available with version 6.12.

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Hello eBPF: Collection of Resources for eBPF (14.5)

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Welcome back to my series on ebpf. In the last post, I told you how to build a Firewall with Java and eBPF. I gave and will give the talk on the very same topic at multiple conferences, as mentioned at the end of the post. Last week, I gave it, together with Mohammed Aboullaite, at one of my favorite Java conferences: JavaZone. One of the reasons I recommend this conference to every upcoming speaker is that they create really good recordings of every talk and upload them to Vimeo almost immediately. So here is the recording of the talk:

You can find the slides here.

As a courtesy to the listener, I created a list of resources on eBPF that helped me a lot:

Main

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