As communication and I/O traffic increase on the interconnection network of high-performance systems, network contention becomes a critical problem drastically reducing performance. Whereas earlier allocation strategies were either sensitive to communication alone or sensitive to I/O alone, we present a new strategy that is sensitive to both communication and I/O. Our new strategy, MC-Elongated, strives to achieve (1) the compactness needed to minimize communication-based contention as well as (2) the balance and orientation relative to I/O nodes needed to minimize I/O-based contention.We tested our new strategy using synthetic workloads and a real workload trace of 6087 jobs captured from a 400 node Intel Paragon. Our results show that with respect to system throughput and average job turnaround time, in environments with varying degree of communication and I/O traffic, MC-Elongated outperforms previous allocation strategies that are in use today. Regarding the tension between communication and I/O, our results show that spatial layout is more critical for I/O-intensive jobs at lower utilization levels and more critical for communication-intensive jobs at higher utilization levels; and that in general, the impact of I/O traffic is dominant.