Eliminating on-chip traffic waste: are we there yet?

While many techniques have been shown to be successful at reducing the amount of on-chip network traffic, no studies have shown how close a combined approach would come to eliminating all unnecessary data traffic, nor have any studies provided insight into where the remaining challenges are. This paper systematically analyzes the traffic inefficiencies of a directory-based MESI protocol and a more efficient hardware-software co-designed protocol, DeNovo. We categorize data waste into various categories and explore several simple optimizations extending DeNovo with the aim of eliminating all of the on-chip network traffic waste. With all the proposed optimizations, we are able to completely eliminate (100%) onchip network traffic waste at L2 for some of the applications (93.5% on average) compared to the previous DeNovo protocol.