1. In a parallel computing environment comprising a network of SMP nodes each having at least one processor, a parallel-aware co-scheduling method for improving the performance and scalability of a dedicated parallel job having synchronizing collective operations, comprising: coordinating interfering system overhead activities on a node and across nodes to promote intra-node and inter-node overlap of said interfering system overhead activities as well as intra-node and inter-node overlap of said synchronizing collective operations, said coordinating step including obtaining and using a globally synchronized time to schedule timer decrement interrupts at the same time across all processors and across all nodes and determine “compute time slots” and time slots for said interfering “system overhead” activities which are respectively synchronized across all processors and across all nodes, and changing the dispatching priority of tasks in each node at the beginning and end of said time slots; and generating timer decrement interrupts, which are a type of interfering system overhead activity, based on a big tick constant greater than 1 as a counter in the kernel where tick processing is involved, for increasing the interval between timer decrement interrupts and thereby reducing the total amount of system overhead cycles.