[virtuabhi]

Single hardware with many cores does not give the same performance as multiple machines. For example, consider disk throughput. If the data is striped across multiple nodes then the read request can be executed in parallel, resulting in linear speed up! In a single machine you have issues of cache misses, inefficient scatter-gather operations in main memory, etc.

And it is much more easier to let the MapReduce framework handle parallelism than writing error prone code with locks/threads/mpi/architecture-dependent parallelism etc.