Datacenters　consolidate　diverse　applications　to　improve　utilization.　However　when　multiple　applications　are　co-located　on　such　platforms,　contention　for　shared　resources　like　Networks-on-Chip　(NoCs)　can　degrade　the　performance　of　latency-critical　online　services　(high-priority　applications).　Recently　proposed　bufferless　NoCs　have　the　advantages　of　requiring　less　area　and　power,　but　they　pose　challenges　in　quality-of-service　(QoS)　support,　which　usually　relies　on　buffer-based　virtual　channels　(VCs).　We　propose　QBLESS,　a　QoS-aware　bufferless　NoC　scheme　for　datacenters.　QBLESS　consists　of　two　components:　a　routing　mechanism　(QBLESS-R)　that　can　substantially　reduce　flit　deflection　for　high-priority　applications,　and　a　congestion-control　mechanism　(QBLESS-CC)　that　guarantees　performance　for　high-priority　applications　and　improves　overall　system　throughput.　We　use　trace-driven　simulation　to　model　a　64-core　system,　finding　that　when　compared　to　BLESS,　a　previous　state-of-the-art　bufferless　NoC　design,　QBLESS　improves　performance　of　high-priority　applications　by　an　average　of　33.2%.