Frequent value locality and value-centric data cache design

Abstract

By studying the behavior of programs in the SPECint95 suite we observed that six out of eight programs exhibit a new kind of value locality, the frequent value locality, according to which a few values appear very frequently in memory locations and are therefore involved in a large fraction of memory accesses. In these six programs ten distinct values occupy over 50% of all memory locations and on an average account for nearly 50% of all memory accesses during program execution. This observation holds for smaller blocks of consecutive memory locations and the set of frequent values remains quite stable over the execution of the program.In the six benchmarks with frequent value locality, on an average 50% of all cache misses occur during the reading or writing of the ten most frequently accessed values. We propose a new data cache structure, the frequent value cache (FVC), which employs a value-centric approach to caching data locations for exploiting the frequent value locality phenomenon. FVC is a small direct-mapped cache which is dedicated to holding only frequently occurring values. The value-centric nature of FVC enables us to store data in a compressed form where the compression is achieved by encoding the frequent values using a few bits. Moreover this simple compression scheme preserves the random access to data values in a cache line.Our experiments demonstrate that by augmenting a direct mapped cache (DMC) with a direct mapped FVC of size no more than 3 Kbytes we can obtain reductions in miss rates ranging from 1% to 68%. In fact we observed that higher reductions in miss rates can he achieved by augmenting a DMC with a small FVC as opposed to doubling the size of DMC for the 124.m88ksim and 134.perl benchmarks.