Mastering SIMD with Java Vector API
Daniel Lemire's blog
I write a lot about data parallelism. About how our processors have special instructions that can do several operations at once. They are often critical to get the best performance out of a CPU.
Compilers are sometimes able to take advantage of these instructions. But it is hard to tell when it will work.
For Java programmers, you can write directly for data parallelism since JDK 16. With enough skill, you might multiply the performance of a few key functions.
The Vector API is in the incubator module jdk.incubator.vector. It still not officially supported, but you can enable it by passing --add-modules=jdk.incubator.vector. I expect that it will soon become a mainstream feature in Java.
Roman Snytsar wrote a book about it: Mastering SIMD with Java Vector API, and the subtitle is Unlocking Single-Core Performance Through SIMD Optimization. I had the honor of being the technical reviewer. I learned a few things while reading the book and I enjoyed it.
Who has time for books, especially technical books?
What a book offers is time to reflect. Reading technical books today is probably just as relevant as it ever was. The author takes you on a story… and gets you to think.
Snytsar’s book works from problems like sum an array, compute a mean and a standard deviation, remove duplicates, merge two sorted arrays, and so forth. A lot of them are similar to the problems you will encounter as a programmer. He starts from the ordinary solution and rebuilds it with the Java Vector API. He then runs benchmarks. He looks at the assembly code.
The book covers performance issues such as unrolling, asymmetric loops, structural hazards, dependencies. Snytsar shows us that, in some instances, data parallelism can fail to bear fruits. The negative lessons are just as important as the positive ones.
The Java Vector API is a thick layer of abstractions, but Snytsar shows that some tricks work better on some hardware than others. I liked the chapter on removing duplicates, which is built on compress. It is one instance where the specific hardware matters. And you are unlikely to just find out about it yourself.
It is a book worth buying if you are a Java programmer with a focus on performance.
The book: Roman Snytsar, Mastering SIMD with Java Vector API, Apress, 2026.
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