In chip design today and for a foreseeable future, the last-level cache and on-chip interconnect is not only performance critical but also a substantial power consumer. This work focuses on employing dynamic voltage and frequency scaling (DVFS) policies for networks-on-chip (NoC) and shared, distributed last-level caches (LLC). In particular, we consider a practical system architecture where the distributed LLC and the NoC share a voltage/frequency domain that is separate from the core domain. This architecture enables the control of the relative speed between the cores and memory hierarchy without introducing synchronization delays within the NoC. DVFS for this architecture is more complex than individual link/core-based DVFS since it involves spatially distributed monitoring and control. We propose an average memory access time (AMAT)-based monitoring technique and integrate it with DVFS based on PID control theory. Simulations on PARSEC benchmarks yield a 27% energy savings with a negligible impact on system performance.