1. A system for maintaining coherence of cache lines in a shared memory multiplexor system comprising: a system area network; and a plurality of compute nodes connected to said system area network, wherein each of said compute nodes includes a main memory, a shared cache and a coherence controller, wherein said coherence controller includes shadow directories, each corresponding to shared caches of other compute nodes connected to said system area network, wherein each of said shadow directories includes state information of said main memory cached in said shared caches of said other compute nodes.