Compiler-directed channel allocation for saving power in on-chip networks

Increasing complexity in the communication patterns of embedded applications parallelized over multiple processing units makes it difficult to continue using the traditional bus-based on-chip communication techniques. The main contribution of this paper is to demonstrate the importance of compiler technology in reducing power consumption of applications designed for emerging multi-processor, NoC (Network-on-Chip) based embedded systems. Specifically, we propose and evaluate a compiler-directed approach to NoC power management in the context of array-intensive applications, used frequently in embedded image/video processing. The unique characteristic of the compiler-based approach proposed in this paper is that it increases the idle periods of communication channels by reusing the same set of channels for as many communication messages as possible. The unused channels in this case take better advantage of the underlying power saving mechanism employed by the network architecture. However, this channel reuse optimization should be applied with care as it can hurt performance if two or more simultaneous communications are mapped onto the same set of channels. Therefore, the problem addressed in this paper is one of reducing the number of channels used to implement a set of communications without increasing the communication latency significantly. To test the effectiveness of our approach, we implemented it within an optimizing compiler and performed experiments using twelve application codes and a network simulation environment. Our experiments show that the proposed compiler-based approach is very successful in practice and works well under both hardware based and software based channel turn-off schemes.