tokens: Garbage collection Kyong2017,Ousterhout2017 , Java virtual machine ( JVM ) positioning to cores Kyong2017 , delays introduced while the tasks move from the scheduler to execution Gittens2016 , disk I / O during shuffling , Java 's just - in - time compilation Ousterhout2017 , output skew Ousterhout2017 , high CPU utilization , disk utilization , unhandled I / O access requests , and network package loss Garraghan2016 were also among other external factors that might introduce stragglers .