TRB: Tag Replication Buffer for Enhancing the Reliability of the Cache Tag Array

Abstract

Protecting the on-chip cache memories against soft errors has become an increasing challenge in designing new generation reliable microprocessors. Previous efforts have mainly focused on improving the reliability of the cache data arrays. Due to its crucial importance to the correctness of cache accesses, the tag array demands high reliability against soft errors while the data array is fully protected. Exploiting the address locality of memory accesses, we propose to duplicate most recently accessed tag entries in a small Tag Replication Buffer (TRB) thus to protect the information integrity of the tag array in the data cache with low performance, energy and area overheads. A Selective-TRB scheme is further proposed to protect only tag entries of dirty cache lines. The experimental results show that the Selective-TRB scheme achieves a higher access-with-replica (AWR) rate of 97.4% for the dirty-cache line tags. To provide a comprehensive evaluation of the tag-array reliability, we also conduct an architectural vulnerability factor (AVF) analysis for the tag array and propose a refined metric, detected-without-replica-AVF (DOR-AVF), which combines the AVF and AWR analysis. Based on our DOR-AVF analysis, a TRB scheme with early write-back (EWB) is proposed, which achieves a zero DOR-AVF at a negligible performance overhead.