That's OK, that is not the issue. Something is deliberately setting MaxNewSize low. With a 6 GB heap then I see this in my gc.log ... -XX:MaxNewSize=174485504. That seems crazy low to me but it wasn't put there by the JVM option fairy, so there must be a reason.
If I add -XX:MaxNewSize=2147483648 to my jvm.options then it is respected. I still get terrible performance from ParNew, just a lot less frequently.
Personally I run logstash using the G1 garbage collector, but I believe that is unsupported by Elastic.