The VM currently starts a large amount of GC threads on systems with many CPUs regardless of the Java heap size.
This doesn't make sense for very small heap sizes.

The number of ParallelGCThreads which currently really get started can be observed by e.g.:
jdk/bin/java -Xmx128m -XX:+PrintFlagsFinal -version|grep GCThreads

The existing option UseDynamicNumberOfGCThreads should be enabled by default to prevent more threads than needed from getting started.