Java 8 automatically using multicore?

Question

I did some tests a year ago concerning multicore with java 7. First I implemented some calculations only in the main thread (CPU usage showed that only one core did all the work

Answer 1

Java 8 does not automatically distribute the work on all CPU cores, unless your code requests it explicitly (for example by using parallel streams).

In some special cases the Hotspot compiler will auto-vectorize the code, see for example JDK-6340864. However, automatic vectorization is using special SIMD CPU instructions, not multiple CPUs.

Also see these answers:

• Does the JVM have the ability to detect opportunities for parallelization?
• Automatic parallelization

(Note that I rewrote the answer, removing the part which was corrected by the comments)

Answer 2

I have experienced this situation too. In my case, it is the frequent garbage collection that causes a significant high (98%) CPU usage. As pointed out by several comments under the original questions, the java GC uses multi-threading by default.

In my opinion, your program is not CPU bound. Therefore, it is not likely to fully use 4 cores. On the other hand, when there is not enough heap space allocated for your program, the frequent GC will easily exhaust the CPU resource. After increasing the heap size, everything is just fine.