Reducing energy of virtual cache synonym lookup using bloom filters

Virtual caches are employed as L1 caches of both high performance and embedded processors to meet their short latency requirements. However, they also introduce the synonym problem where the same physical cache line can be present at multiple locations in the cache due to their distinct virtual addresses, leading to potential data consistency issues. To guarantee correctness, common hardware solutions either perform serial lookups for all possible synonym locations in the L1 consuming additional energy or employ a reverse map in the L2 cache that incurs a large area overhead. Such preventive mechanisms are nevertheless indispensable even though synonyms may not always be present during the execution.In this paper, we study the synonym issue using Windows applications workload and propose a technique based on Bloom filters to reduce synonym lookup energy. By tracking the address stream using Bloom filters, we can confidently exclude the addresses that were never observed to eliminate unnecessary synonym lookups, thereby saving energy in the L1 cache. Bloom filters have a very small area overhead making our implementation a feasible and attractive solution for synonym detection. Our results show that synonyms in these applications actually constitutes less than 0.1% of the total cache misses. By applying our technique, the dynamic energy consumed in L1 data cache can be reduced up to 32.5%. When taking leakage energy into account, the savings is up to 27.6%.