Collecting data with streams - Java 8 in Action: Lambdas, Streams, and Functional-Style Programming - page 181

Figure 6.8. Parallelizing the reduction process using the combiner
method
 The original stream is recursively split in substreams until a condition defining whether a stream
needs to be further divided becomes false (parallel computing is often slower than sequential
computing when the units of work being distributed are too small, and it's pointless to generate many
more parallel tasks than you have processing cores).
 At this point all substreams can be processed in parallel, each of them using the sequential reduction
algorithm shown in figure 6.7 .