Archipelago: A polymorphic cache design for enabling robust near-threshold operation

Extreme technology integration in the sub-micron regime comes with a rapid rise in heat dissipation and power density for modern processors. Dynamic voltage scaling is a widely used technique to tackle this problem when high performance is not the main concern. However, the minimum achievable supply voltage for the processor is often bounded by the large on-chip caches since SRAM cells fail at a significantly faster rate than logic cells when reducing supply voltage. This is mainly due to the higher susceptibility of the SRAM structures to process-induced parameter variations. In this work, we propose a highly flexible fault-tolerant cache design, Archipelago, that by reconfiguring its internal organization can efficiently tolerate the large number of SRAM failures that arise when operating in the near-threshold region. Archipelago partitions the cache to multiple autonomous islands with various sizes which can operate correctly without borrowing redundancy from each other. Our configuration algorithm - an adapted version of minimum clique covering - exploits the high degree of flexibility in the Archipelago architecture to reduce the granularity of redundancy replacement and minimize the amount of space lost in the cache when operating in near-threshold region. Using our approach, the operational voltage of a processor can be reduced to 375mV, which translates to 79% dynamic and 51% leakage power savings (in 90nm) for a microprocessor similar to the Alpha 21364. These power savings come with a 4.6% performance drop-off when operating in low power mode and 2% area overhead for the microprocessor.