The expression big data does not mean a specific amount of data, neither in the number of examples nor in the number of gigabytes, terabytes, or petabytes occupied by the data. It means that data has been growing faster than processing power. This implies the following:

• Some of the methods and techniques that worked well in the past now need to be redone or replaced as they do not scale well to the new size of the input data
• Algorithms cannot assume that all the input data can fit in memory
• Managing data becomes a major task in itself
• Using computer clusters or multicore machines becomes a necessity and not a luxury

This chapter will focus on this last piece of the puzzle: how to use multiple cores (either on the same machine or on separate machines) to speed up and organize your computations. This will also be useful in other medium-sized data tasks.