Umit Y. Ogras

Outstanding Research Problems in NoC Design: System, Microarchitecture, and Circuit Perspectives

To alleviate the complex communication problems that arise as the number of on-chip components increases, network-on-chip (NoC) architectures have been recently proposed to replace global interconnects. In this paper, we first provide a general description of NoC architectures and applications. Then, we enumerate several related research problems organized under five main categories: Application characterization, communication paradigm, communication infrastructure, analysis, and solution evaluation. Motivation, problem description, proposed approaches, and open issues are discussed for each problem from system, microarchitecture, and circuit perspectives. Finally, we address the interactions among these research problems and put the NoC design process into perspective.

It's a small world after all: NoC performance optimization via long-range link insertion

Networks-on-chip (NoCs) represent a promising solution to complex on-chip communication problems. The NoC communication architectures considered so far are based on either completely regular or fully customized topologies. In this paper, we present a methodology to automatically synthesize an architecture which is neither regular nor fully customized. Instead, the communication architecture we propose is a superposition of a few long-range links and a standard mesh network. The few application-specific long-range links we insert significantly increase the critical traffic workload at which the network transitions from a free to a congested state. This way, we can exploit the benefits offered by both complete regularity and partial topology customization. Indeed, our experimental results demonstrate a significant reduction in the average packet latency and a major improvement in the achievable network through with minimal impact on network topology

Key research problems in NoC design: a holistic perspective

Networks-on-Chip (NoCs) have been recently proposed as a promising solution to complex on-chip communication problems. The lack of an unified representation of applications and architectures makes NoC problem formulation and classification both difficult and obscure. To remedy this situation, we provide a general description for NoC architectures and applications and then enumerate several outstanding research problems (denoted by P1-P8) organized under three topics: communication infrastructure synthesis, communication paradigm selection, and application mapping optimization. Far from being exhaustive, the discussed problems are deemed essential for future NoC research.

On-chip communication architecture exploration: A quantitative evaluation of point-to-point, bus, and network-on-chip approaches

Traditionally, design-space exploration for systems-on-chip (SoCs) has focused on the computational aspects of the problem at hand. However, as the number of components on a single chip and their performance continue to increase, a shift from computation-based to communication-based design becomes mandatory. As a result, the communication architecture plays a major role in the area, performance, and energy consumption of the overall system. This article presents a comprehensive evaluation of three on-chip communication architectures targeting multimedia applications. Specifically, we compare and contrast the network-on-chip (NoC) with point-to-point (P2P) and bus-based communication architectures in terms of area, performance, and energy consumption. As the main contribution, we present complete P2P, bus-, and NoC-based implementations of a real multimedia application (i. e. the MPEG-2 encoder), and provide direct measurements using an FPGA prototype and actual video clips, rather than simulation and synthetic workloads. We also support the experimental findings through a theoretical analysis. Both experimental and analysis results show that the NoC architecture scales very well in terms of area, performance, energy, and design effort, while the P2P and bus-based architectures scale poorly on all accounts except for performance and area, respectively.

System-Level Buffer Allocation for Application-Specific Networks-on-Chip Router Design

In this paper, a novel system-level buffer planning algorithm that can be used to customize the router design in networks-on-chip (NoCs) is presented. More precisely, given the traffic characteristics of the target application and the total budget of the available buffering space, the proposed algorithm automatically assigns the buffer depth for each input channel, in different routers across the chip, such that the overall performance is maximized. This is in deep contrast with the uniform assignment of buffering resources (currently used in NoC design), which can significantly degrade the overall system performance. Indeed, the experimental results show that while the proposed algorithm is very fast, significant performance improvements can be achieved compared to the uniform buffer allocation. For instance, for a complex audio/video application, about 80% savings in buffering resources, can be achieved by smart buffer allocation using the proposed algorithm

Energy- and Performance-Aware Incremental Mapping for Networks on Chip With Multiple Voltage Levels

Achieving effective run-time mapping on multiprocessor systems-on-chip (MPSoCs) is a challenging task, particularly since the arrival order of the target applications is not known a priori. This paper targets real-time applications which are dynamically mapped onto embedded MPSoCs, where communication happens via the Network-on-Chip (NoC) approach, and resources connected to the NoC have multiple voltage levels. We address precisely the energy- and performance-aware incremental mapping problem for NoCs with multiple voltage levels and propose an efficient technique (consisting of region selection and node allocation) to solve it. Moreover, the proposed technique allows for new applications to be added to the system with minimal in- terprocessor communication overhead. Experimental results show that the proposed technique is very fast, and as much as 50% communication energy savings can be achieved compared to using an arbitrary allocation scheme.

Voltage-frequency island partitioning for GALS-based networks-on-chip

Due to high levels of integration and complexity, the design of multi-core SoCs has become increasingly challenging. In particular, energy consumption and distributing a single global clock signal throughout a chip have become major design bottlenecks. To deal with these issues, a globally asynchronous, locally synchronous (GALS) design is considered for achieving low power consumption and modular design. Such a design style fits nicely with the concept of voltage-frequency islands (VFIs) which has been recently introduced for achieving fine-grain system-level power management. This paper proposes a design methodology for partitioning an NoC architecture into multiple VFIs and assigning supply and threshold voltage levels to each VFI Simulation results show about 40% savings for a real video application and demonstrate the effectiveness of our approach in reducing the overall system energy consumption. The results and functional correctness are validated using an FPGA prototype for an NoC with multiple VFIs.

Application-specific network-on-chip architecture customization via long-range link insertion

Networks-on-chip (NoCs) represent a promising solution to complex on-chip communication problems. The NoC communication architectures considered so far are based on either completely regular or fully customized topologies. In this paper, we present a methodology to automatically synthesize an architecture where a few application-specific long-range links are inserted on top of a regular mesh network. This way, we can better exploit the benefits of both complete regularity and partial customization. Indeed, our experimental results show that inserting application-specific long-range links significantly increases the critical traffic workload at which the network state transits from a free to a congested regime. This, in turn, results in a significant reduction in the average packet latency and a major improvement in the network achievable throughput.

Energy- and Performance-Driven NoC Communication Architecture Synthesis Using a Decomposition Approach

In this paper, we present a methodology for customized communication architecture synthesis that matches the communication requirements of the target application. This is an important problem, particularly for network-based implementations of complex applications. Our approach is based on using frequently encountered generic communication primitives as an alphabet capable of characterizing any given communication pattern. The proposed algorithm searches through the entire design space for a solution that minimizes the system total energy consumption, while satisfying the other design constraints. Compared to the standard mesh architecture, the customized architecture generated by the newly proposed approach shows about 36% throughput increase and 51% reduction in the energy required to encrypt 128 bits of data with a standard encryption algorithm.

Design and Management of Voltage-Frequency Island Partitioned Networks-on-Chip

The design of many core systems-on-chip (SoCs) has become increasingly challenging due to high levels of integration, excessive energy consumption and clock distribution problems. To deal with these issues, we consider network-on-chip (NoC) architectures partitioned into several voltage-frequency islands (VFIs) and propose a design methodology for runtime energy management. The proposed approach minimizes the energy consumption subject to performance constraints. Then, we present efficient techniques for on-the-fly workload monitoring and management to ensure that the system can cope with variability in the workload and various technology-related parameters. Simulation results demonstrate the effectiveness of our approach in reducing the overall system energy consumption for a real video application. Finally, the results and functional correctness are validated using an field-programmable gate-array (FPGA) prototype for an NoC with multiple VFIs.