I’m an OpenJDK Reviewer mainly focused on the Garbage Collection (GC) subsystem(s), with an emphasis on ZGC. You can see my active roles in the OpenJDK Community Census https://openjdk.org/census#jsikstro. Contributions You can find the bul…
An nmethod, which I’ll refer to as NMethod, is HotSpot’s representation of a natively compiled Java method, the result of a JIT compilation, typically produced by the C1 or C2 compilers. NMethods are interesting because they require coopera…
OpenJDK is a large and complex codebase and navigating it efficiently takes years of experience. This post is about how I’ve been using agentic AI workflows to move faster in that environment, what has worked for me, and where I as a human
Cache coherency is notoriously tricky to reason about, and even trickier to debug. With that challenge in mind, we’ll deep dive into cache coherency considerations in the design and development of a JIT compiler I’ve been working on. The JI…
EDIT (March 1, 2026): I updated the title to to clarify that this observation is specific to my environment. The original title may have implied a universal behavior, which isn’t the case. Thanks for the feedback! TLDR; The C++ standard lib…
While digging into Project Valhalla, I encountered a subtle part of the JVM that turned out to be a prime opportunity for optimization. This exploration led to an approach that shaves unused memory from Java class instance metadata. In this…
NUMA-aware relocation is a feature recently added to ZGC, one of the garbage collectors in OpenJDK, with the introduction of JDK-8359683, set to release in JDK 26. Building on the recent memory allocation overhaul (see “How ZGC allocates me…
MADV_COLLAPSE is a madvise(2) advice (added in Linux 6.1) that requests a best-effort, synchronous collapse of native pages in a range into Transparent Huge Pages (THPs). As the man page states: Perform a best-effort synchronous collapse of…
This post explores how ZGC, one of the garbage collectors in the OpenJDK, allocates memory for the Java heap, focusing on enhancements introduced in JDK-8350441 with the Mapped Cache. A garbage collector does much more than just collect gar…
TLDR; If you are experiencing similar issues, upgrade to the latest version of ddclient or change IP-check website to something other than dynamicdns.park-your-domain.com. See Discussion. On the morning of 22/3 I noticed that my website was…
Analyzing performance can be challenging, especially when diagnosing regressions. Finding the right methods to investigate performance issues is often time-consuming and complex. You may have heard of perf, a powerful Linux tool for perform…
When I first found out about huge pages I was a bit confused. Why do we need larger pages? How do I configure this? Is it always on? I had many questions. The first thing you should know is that the concept of larger pages has many names, a…