A parallel section is the part of an algorithm where independent operations can run at the same time. An algorithm may also contain sequential sections that must happen in order, so parallel execution cannot make every task infinitely fast. section 2.11
Searching index cards
Divide a mixed pile of cards among several people, let everyone search independently for cards about concurrency, then combine the findings. Dividing and combining remain sequential; searching is the parallel section. With two helpers, the five-second process falls to three seconds rather than one, because the sequential work still takes time. section 2.11
