Providing task isolation via TLB coloring

Abstract

The translation look aside buffer (TLB) improves the performance of systems by caching the virtual page to physical frame mapping. But TLBs present a source of unpredictability for real-time systems. Standard heap allocated regions do not provide guarantees on the TLB set that will hold a particular page translation. This unpredictability can lead to TLB misses with a penalty of up to thousands of cycles and consequently intra- and inter-task interference resulting in loose bounds on the worst case execution time (WCET) and TLB-related preemption delay. In this work, we design and implement a new heap allocator that guarantees the TLB set, which will hold a particular page translation on a uniprocessor of a contemporary architecture. The allocator is based on the concept of page coloring, a software TLB partitioning method. Virtual pages are colored such that two pages of different color cannot map to the same TLB set. Our experimental evaluations confirm the unpredictability associated with the standard heap allocation. Using a set of synthetic and standard benchmarks, we show that our allocator provides task isolation for real-time tasks. To the best of our knowledge, such TLB isolation without special hardware support is unprecedented, increases TLB predictability and can facilitate WCET analysis.