JVM only uses 4 cores

I have a processor-intensive Java program and I want it to use all 8 available cores on the machine. I've run taskset and it reports the mask to be: pid 5897's current affinity mask: ff But in top (when running 12 (heavy) threads): 5897 ubuntu 20 0 9099664 1.7g 18608 S 400.7 11.1 25:52.83 java What's going on here?