Trivia problem: Generate primes between 1 and 100 million with Hadoop MapReduce job.
Input: Numbers from 1 to 100 million, 1 per line - use a simple C program for this.
Output: Emit all the prime numbers.

Execution time: ~2 minutes and 20 seconds [i.e. 140 seconds] in hdfs-local mode on my Cloudera-Udacity-Hadoop VM [VM has been given 3 GB RAM].
  real    2m15.895s
  user    0m3.048s
  sys     0m0.517s

Part files: 1
$ ls
part-r-00000

$ tailf part-r-00000
16147529
16147531
16147561
16147591
16147597
16147609
16147613
16147627
16147639
16147643

[PB] $ more part-r-00000 
2
3
5
7
11
13
17
19
23
29
31
37
41
43
47
53
59
61
67
71
73
79
83
89
97

How many primes between 1 and 100 million?
$ wc -l part-r-00000 
5761455 part-r-00000

Only 5.76 million primes between 1 to 100 million.

In the same environment, a pure Java program took 5 minutes and 26 seconds [i.e. 326 seconds] for computing prime numbers between 1 and 100 million [same as above].