Akka - during load testing, forkjoinpool.scan at 20% of cpu time

Akka - during load testing, forkjoinpool.scan at 20% of cpu time

We're making some good progress in load testing and scaling an akka
application but we're seeing scala.concurrent.forkjoin.ForkJoinPool.scan()
coming up as the second highest hotspot around 20% of self time in
visualvm. The Self time (CPU) column says only a fraction of that (say
.05%).
I suspect this means blocking or context switching are potentially
problematic but I'm not too sure - can any one give insight? If it's
context switching I'm guessing tuning dispatcher throughput to a higher
number may net us gains, otherwise if it's caused by blocking we'll need
to read through the code some more.
Any insight greatly appreciated.