1. An aggregate symmetric multiprocessor (SMP) data processing system, comprising: multiple SMP computers including a first SMP computer having at least first and second processing units and a first system memory pool, a second SMP computer having at least third and fourth processing units and a second system memory pool, and a third SMP computer having at least fifth and sixth processing units and third, fourth and fifth system memory pools, wherein: the third system memory pool is accessible to both the fifth and sixth processing units; the fourth system memory pool is a restricted access memory pool inaccessible to the third, fourth and sixth processing units and accessible to at least the second and fifth processing units; the fifth system memory pool is a restricted access memory pool inaccessible to the first, second and sixth processing units and accessible to at least the fourth and fifth processing units; and an interconnect fabric including a first interconnect coupling the second processing unit in the first SMP computer for load-store coherent, ordered access to the fourth system memory pool in the third SMP computer and a second interconnect coupling the fourth processing unit in the second SMP computer for load-store coherent, ordered access to the fifth system memory pool in the third SMP computer, wherein the second processing unit in the first SMP computer and the fourth system memory pool in the third SMP computer form a synthetic fourth SMP computer and the fourth processing unit in the second SMP computer and the fifth system memory pool in the third SMP computer form a synthetic fifth SMP computer; wherein the first SMP computer system and the third SMP computer system employ hardware transaction tag aliasing such that a first component in the first SMP computer system and a second component in the third SMP computer system that are not within the fourth SMP computer system append a shared hardware transaction tag to memory access transactions.