Centaur: a hybrid network-on-chip architecture utilizing micro-network fusion


The escalating proliferation of multicore chips has accentuated the criticality of the on-chip network. Packet-based networks-on-chip (NoC) have emerged as the de facto interconnect of future chip multi-processors (CMP). On-chip traffic comprises a mixture of data and control messages from the cache coherence protocol. Given the latency-criticality of control messages, in this paper we aim to optimize their delivery times. Instead of treating the on-chip router as a monolithic component, we advocate the introduction of an ultra-low-latency ring-inspired (i.e., utilizing ring primitive building blocks) support micro-network that is optimized for control messages. This \(\upmu \)NoC is fused with a throughput-driven conventional NoC router to form a hybrid architecture, called Centaur, which maintains separate data paths and control logic for the two fused networks. Full-system simulation results from a 64-core CMP indicate that the proposed fused Centaur router improves overall system performance by up to 26 %, as compared to a state-of-the-art router implementation. Furthermore, hardware synthesis results using commercial 65 nm libraries indicate that Centaur’s area and power overheads are 9 and 3 %, respectively, as compared to a baseline router design. More importantly, the new design does not affect the router’s critical path.

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    In Greek mythology, Centaur was a hybrid creature that was part human and part horse. Much like this mythical creature, our proposed router architecture fuses two distinct networks into one entity.

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Correspondence to Junghee Lee.

Appendix I: Formal proof of protocol-level deadlock avoidance

Appendix I: Formal proof of protocol-level deadlock avoidance

As stated in Sect. 4.2, progressive recovery mechanisms resolve all types of deadlocks when the following two conditions are met [6, 31]: (1) the recovery network is free from deadlocks, and (2) in each and every deadlock situation, there exists at least one packet that is granted access to the recovery network. The following theorems prove that these two conditions are satisfied in the Centaur architecture and, thus, any protocol- level deadlocks are guaranteed to be broken by the proposed time-out mechanism.

Theorem 1

The deadlock freedom of the DNoC is not affected by the \(\mu \)NoC.


Although the inter-router links are shared between the DNoC and the \(\upmu \)NoC, the latter has its own buffers (\(\upmu \)NoC Buffer and Intermediate Buffer). Therefore, the \(\upmu \)NoC does not affect the deadlock freedom of the DNoC, because the packets in the \(\upmu \)NoC do not block any packets in the DNoC. \(\square \)

Theorem 2

The time-out mechanism enables every packet in a deadlock situation to have the opportunity to access the recovery network.


When a protocol-level deadlock occurs (or is suspected), all the packets in the \(\upmu \)NoC buffers (\(\upmu \)NoC and Intermediate Buffers) are given the chance to escape from the deadlock. As mentioned in Sect. 4.2, even if a packet is not at the head of the buffer, it is also given the same opportunity. \(\square \)

Corollary 1

All dependencies among the various message classes involved in deadlock situations are broken by the time-out mechanism.


Dependencies among message classes can, indeed, be created in the \(\upmu \)NoC buffers, whereas there are no such dependencies in the DNoC. In the \(\upmu \)NoC, a packet may be blocked by a preceding packet that belongs to a different message class. When the time-out mechanism is triggered, packets are no longer blocked by preceding packets in the \(\upmu \)NoC, because they are forwarded to the DNoC, and the head packet does not block the following packet(s) in the same buffer. \(\square \)

One drawback of this mechanism is that the packet order might be reversed. To preserve packet order, control packets should leave each router in the order of arrival, regardless of which network (\(\upmu \)NoC or DNoC) they arrive through. Packets with the same input-output port mappings should be ordered. As a hardware implementation of packet ordering, we introduce a sequence numbering mechanism within the router. Note that this mechanism is accounted for in the area/power/timing evaluation of Sect. 5.3.

For every pair of input and output ports, two counters are maintained for each VC (message class) in the DNoC, which makes use of control packets. The ‘Head’ counter indicates the order of arriving packets and the ‘Tail’ counter indicates the order of departing packets. When a control packet arrives at a router, it is stored either in the \(\upmu \)NoC Buffer, or the Intermediate Buffer, or a VC buffer within the DNoC. Irrespective of which buffer it is stored into, a sequence number is given. The sequence number is equal to ‘Head’ and the ‘Head’ counter is subsequently increased by one. The packet can leave only when its sequence number matches the ‘Tail’ counter. After the packet departs, ‘Tail’ is increased by one.

However, the sequence numbering raises additional dependencies within a router, which may incur deadlocks. The following theorem proves that deadlocks do not happen.

Theorem 3

Deadlock freedom is not affected by the additional dependencies created by the sequence numbering mechanism.


Corollary 1 proves that there are no dependencies among message classes when the time-out mechanism is triggered. The additional dependencies caused by the sequence numbering mechanism are only within the same message class (VC). Therefore, the additional dependencies do not cause any packet to be blocked by other message classes. In addition, since the dependencies are created based on the order of arrival (i.e., the dependencies are, essentially, ordered), they do not form any cycles. \(\square \)

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Lee, J., Nicopoulos, C., Lee, H.G. et al. Centaur: a hybrid network-on-chip architecture utilizing micro-network fusion. Des Autom Embed Syst 18, 121–139 (2014). https://doi.org/10.1007/s10617-014-9131-z

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  • Networks-on-chip
  • Interconnection networks
  • Segregated/separated networks