[taneq]

If these problems can't explicitly be scaled horizontally by the developer, why would this system be able to scale them horizontally by doing it behind the scenes? Any limitations of data exchange between nodes would still apply.

[dh997]

Most of these single system image clusters have used custom, fast interconnects akin to Inifinband or hypertransport to ameliorate locality issues (latency and bandwidth)... Without such, it's neigh impossible without pinning all resources for a "process" to one physical box.

Good luck ever getting more than one box worth of performance or resources out of a single application instance.

Hence MapReduce, actors, promises, message-passing, etc.