Section: New Results

Fault Tolerance

WCET estimation for architectures with faulty caches

Participants : Damien Hardy, Isabelle Puaut.

Fine-grained disabling and reconfiguration of hardware elements (functional units, cache blocks) will become economically necessary to recover from permanent failures, whose rate is expected to increase dramatically in the near future. This fine-grained disabling will lead to degraded performance as compared to a fault-free execution.

Until recently, all static worst-case execution time (WCET) estimation methods were assuming fault-free processors, resulting in unsafe estimates in the presence of faults. The first static WCET estimation technique dealing with the presence of permanent faults in instruction caches was proposed in [4]. This study probabilistically quantified the impact of permanent faults on WCET estimates. It demonstrated that the probabilistic WCET (pWCET) estimates of tasks increase rapidly with the probability of faults as compared to fault-free WCET estimates.

New results show that very simple reliability mechanisms allow mitigating the impact of faulty cache blocks on pWCETs. Two mechanisms, that make part of the cache resilient to faults are analyzed. Experiments show that the gain in pWCET for these two mechanisms are on average 48 % and 40 % as compared to an architecture with no reliability mechanism.

This work [35] appeared at DATE 2016 (best paper award for the embedded systems track).

This is joint work with Yannakis Sazeides from University of Cyprus.