  With relentless CMOS technology downsizing Networks-on-Chips (NoCs) are
inescapably experiencing escalating susceptibility to wearout and reduced
reliability. While faults in processors and memories may be masked via
redundancy, or mitigated via techniques such as task migration, NoCs are
especially vulnerable to hardware faults as a single link breakdown may cause
inter-tile communication to halt indefinitely, rendering the whole multicore
chip inoperable. As such, NoCs impose the risk of becoming the pivotal point of
failure in chip multicores that utilize them. Aiming towards seamless NoC
operation in the presence of faulty links we propose Hermes, a near-ideal
fault-tolerant routing algorithm that meets the objectives of exhibiting high
levels of robustness, operating in a distributed mode, guaranteeing freedom
from deadlocks, and evening-out traffic, among many. Hermes is a
limited-overhead deadlock-free hybrid routing algorithm, utilizing
load-balancing routing on fault-free paths to sustain high-throughput, while
providing pre-reconfigured escape path selection in the vicinity of faults.
Under such online mechanisms, Hermes's performance degrades gracefully with
increasing faulty link counts, a crucially desirable response lacking in
prior-art. Additionally, Hermes identifies non-communicating network partitions
in scenarios where faulty links are topologically densely distributed such that
packets being routed to physically isolated regions cause no network stagnation
due to indefinite chained blockages starting at sub-network boundaries. An
extensive experimental evaluation, including utilizing traffic workloads
gathered from full-system chip multi-processor simulations, shows that Hermes
improves network throughput by up to $3\times$ when compared against the
state-of-the-art. Further, hardware synthesis results prove Hermes's efficacy.
