Why does stream parallel() not use all available threads?

Question

I tried to run 100 Sleep tasks in parallel using Java8(1.8.0_172) stream.parallel() submitted inside a custom ForkJoinPool with 100+ threads available. Each

Answer 1

Since the Stream implementation’s use of the Fork/Join pool is an implementation detail, the trick to force it to use a different Fork/Join pool is undocumented as well and seems to work by accident, i.e. there’s a hardcoded constant determining the actual parallelism, depending on the default pool’s parallelism. So using a different pool was not foreseen, originally.

However, it has been recognized that using a different pool with an inappropriate target parallelism is a bug, even if this trick is not documented, see JDK-8190974.

It has been fixed in Java 10 and backported to Java 8, update 222.

So a simple solution world be updating the Java version.

You may also change the default pool’s parallelism, e.g.

System.setProperty("java.util.concurrent.ForkJoinPool.common.parallelism", "100");

before doing any Fork/Join activity.

But this may have unintended effects on other parallel operations.