Section: New Results

Reconfigurable Architecture Design

Voltage Over-Scaling for Error-Resilient Applications

Participants : Rengarajan Ragavan, Benjamin Barrois, Cédric Killian, Olivier Sentieys.

Voltage scaling has been used as a prominent technique to improve energy efficiency in digital systems, scaling down supply voltage effects in quadratic reduction in energy consumption of the system. Reducing supply voltage induces timing errors in the system that are corrected through additional error detection and correction circuits. In [43], we proposed voltage over-scaling based approximate operators for applications that can tolerate errors. We characterized the basic arithmetic operators using different operating triads (combination of supply voltage, body-biasing scheme and clock frequency) to generate models for approximate operators. Error-resilient applications can be mapped with the generated approximate operator models to achieve optimum trade-off between energy efficiency and error margin. Based on the dynamic speculation technique, best possible operating triad is chosen at runtime based on the user definable error tolerance margin of the application. In our experiments in 28nm FDSOI, we achieved maximum energy efficiency of 89% for basic operators like 8-bit and 16-bit adders at the cost of 20% Bit Error Rate (ratio of faulty bits over total bits) by operating them in near-threshold regime.

Stochastic Computation Elements with Correlated Input Streams

Participants : Rengarajan Ragavan, Rahul Kumar Budhwani, Olivier Sentieys.

In recent years, shrinking size in integrated circuits has imposed a big challenge in maintaining the reliability in conventional computing. Stochastic Computing (SC) has been seen as a reliable, low-cost, and low-power alternative to overcome such issues. SC computes data in the form of bit streams of 1s and 0s. Therefore, SC outperforms conventional computing in terms of tolerance to soft error and uncertainty at the cost of increased computational time. Stochastic Computing with uncorrelated input streams requires streams to be highly independent for better accuracy. This results in more hardware consumption for conversion of binary numbers to stochastic streams. Correlation can be used to design Stochastic Computation Elements (SCE) with correlated input streams. These designs have higher accuracy and less hardware consumption. In [38], we proposed new SC designs to implement image processing algorithms with correlated input streams. Experimental results of proposed SC with correlated input streams show on average 37% improvement in accuracy with reduction of 50-90% in area and 20-85% in delay over existing stochastic designs.

Fault Tolerant Architectures

Participants : Olivier Sentieys, Angeliki Kritikakou, Rafail Psiakis.

Error occurrence in embedded systems has significantly increased, whereas critical applications require reliable processors that combine performance with low cost and energy consumption. Very Long Instruction Word (VLIW) processors have inherent resource redundancy which is not constantly used due to application's fluctuating Instruction Level Parallelism (ILP). Approaches can benefit these additional resources to provide fault tolerance.

The reliability through idle slots utilization can be explored either at compile-time, increasing code size and storage requirements, or at run-time only inside the current instruction bundle, adding unnecessary time slots and degrading performance. To address this issue, we proposed a technique in [41] to explore the idle slots inside and across original and replicated instruction bundles reclaiming more efficiently the idle slots and creating a compact schedule. To achieve this, a dependency analysis is applied at run-time. The execution of both original and replicated instructions is allowed at any adequate function unit, providing higher flexibility on instruction scheduling. The proposed technique achieves up to 26% reduction in performance degradation over existing approaches.

When permanent and soft errors coexist, spare units have to be used or the executed program has to be modified through self-repair or by using several stored versions. However, these solutions introduce high area overhead for the additional resources, time overhead for the execution of the repair algorithm and storage overhead of the multi-versioning. To address these limitations, a hardware mechanism is proposed in [42] which at run-time replicates the instructions and schedules them at the idle slots considering the resource constraints. If a resource becomes faulty, the proposed approach efficiently rebinds both the original and replicated instructions during execution. In this way, the area overhead is reduced, as no spare resources are used, whereas time and storage overhead are not required. Results show up to 49% performance gain over existing techniques.

Hardware Accelerated Simulation of Heterogeneous Platforms

Participants : Minh Thanh Cong, François Charot, Steven Derrien.

When considering designing heterogeneous multi-core platforms, the number of possible design combinations leads to a huge design space, with subtle trade-offs and design interactions. To reason about what design is best for a given target application requires detailed simulation of many different possible solutions. Simulation frameworks exist (such as gem5) and are commonly used to carry out these simulations. Unfortunately, these are purely software-based approaches and they do not allow a real exploration of the design space. Moreover, they do not really support highly heterogeneous multi-core architectures. These limitations motivate the study of the use of hardware to accelerate the simulation, and in particular of FPGA components. In this context, we are currently investigating the possibility of building hardware accelerated simulators using the HAsim simulation infrastructure, jointly developed by MIT and Intel. HAsim is an FPGA-accelerated simulator that is able to simulate a multicore with a high-detailed pipeline, cache hierarchy and detailed on-chip network on a single FPGA. A model of the RISC-V instruction set architecture suited to the HAsim infrastructure has been developed, its deployment on the Xeon+FPGA Intel platform is in progress. This work is done with the perspective of studying hardware accelerated simulation of heterogeneous multicore architectures mixing RISC-V cores and hardware accelerators.

Optical Interconnections for 3D Multiprocessor Architectures

Participants : Jiating Luo, Ashraf El-Antably, Van Dung Pham, Cédric Killian, Daniel Chillet, Olivier Sentieys.

To address the issue of interconnection bottleneck in multiprocessor on a single chip, we study how an Optical Network-on-Chip (ONoC) can leverage 3D technology by stacking a specific photonics die. The objectives of this study target: i) the definition of a generic architecture including both electrical and optical components, ii) the interface between electrical and optical domains, iii) the definition of strategies (communication protocol) to manage this communication medium, and iv) new techniques to manage and reduce the power consumption of optical communications. The first point is required to ensure that electrical and optical components can be used together to define a global architecture. Indeed, optical components are generally larger than electrical components, so a trade-off must be found between the size of optical and electrical parts. For the second point, we study how the interface can be designed to take applications needs into account. From the different possible interface designs, we extract a high-level performance model of optical communications from losses induced by all optical components to efficiently manage Laser parameters. Then, the third point concerns the definition of high-level mechanisms which can handle the allocation of the communication medium for each data transfer between tasks. This part consists in defining the protocol of wavelength allocation. Indeed, the optical wavelengths are a shared resource between all the electrical computing clusters and are allocated at run time according to application needs and quality of service. The last point concerns the definition of techniques allowing to reduce the power consumption of on-chip optical communications. The power of each Laser can be dynamically tuned in the optical/electrical interface at run time for a given targeted bit-error-rate. Due to the relatively high power consumption of such integrated Laser, we study how to define adequate policies able to adapt the laser power to the signal losses.

In [37] we designed an Optical-Network-Interface (ONI) to connect a cluster of several processors to the optical communication medium. This interface, constrained by the 10 Gb/s data-rate of the Lasers, integrates Error Correcting Codes (ECC) and a communication manager. This manager can select, at run-time, the communication mode to use depending on timing or power constraints. Indeed, as the use of ECC is based on redundant bits, it increases the transmission time, but saves power for a given Bit Error Rate (BER). Moreover, our ONI allows for data to be sent using several wavelengths in parallel, hence increasing transmission bandwidth. From the design of this interface, estimation in terms of power consumption and execution time have been obtained, as well as the energy per bit of each communication.

The optical medium can support multiple transactions at the same time on different wavelengths by using Wavelength Division Multiplexing (WDM). Moreover, multiple wavelengths can be gathered as high-bandwidth channel to reduce transmission time. However, multiple signals sharing simultaneously a waveguide lead to inter-channel crosstalk noise. This problem impacts the Signal to Noise Ratio (SNR) of the optical signal, which increases the Bit Error Rate (BER) at the receiver side. In [39], we formulated the crosstalk noise and execution time models and then proposed a Wavelength Allocation (WA) method in a ring-based WDM ONoC to reach performance and energy trade-offs based on the application constraints. We showed that for a 16-core ONoC architecture using 12 wavelengths, more than ${10}^5$ allocation solutions exist and only 51 are on a Pareto front giving a tradeoff between execution time and energy per bit (derived from the BER). These optimized solutions reduce the execution time by 37% or the energy from 7.6fJ/bit to 4.4fJ/bit.

We also proposed to explore the selection of laser power for each communication. This approach reduces the global power consumption by ensuring the targeted Bit Error Rate for each communication. To support laser power selection, we have also studied, designed and evaluated at transistor level different configurable laser drivers using a 28NM FDSOI technology.

Adaptive Dynamic Compilation for Low-Power Embedded Systems

Participants : Steven Derrien, Simon Rokicki.

Single ISA-Heterogeneous multi-cores such as the ARM big.LITTLE have proven to be an attractive solution to explore different energy/performance trade-offs. Such architectures combine Out of Order cores with smaller in-order ones to offer different power/energy profiles. They however do not really exploit the characteristics of workloads (compute-intensive vs. control dominated).

In this work, we propose to enrich these architectures VLIW cores, which are very efficient at compute-intensive kernels. To preserve the single ISA programming model, we resort to Dynamic Binary Translation as used in Transmeta Crusoe and NVidia Denver processors. Our proposed DBT framework targets the RISC-V ISA, for which both OoO and in-order implementations exist.

Since DBT operates at runtime, its execution time is directly perceptible by the user, hence severely constrained. As a matter of fact, this overhead has often been reported to have a huge impact on actual performance, and is considered as being the main weakness of DBT based solutions. This is particularly true when targeting a VLIW processor: the quality of the generated code depends on efficient scheduling; unfortunately scheduling is known to be the most time-consuming component of a JIT compiler or DBT. Improving the responsiveness of such DBT systems is therefore a key research challenge. This is however made very difficult by the lack of open research tools or platform to experiment with such platforms.

To address these issues, we have developed an open hardware/software platform supporting DBT. The platform was designed using HLS tools and validated on a FPGA board. The DBT uses RISC-V as host ISA, and can be retargeted to different VLIW configurations. Our platform uses custom hardware accelerators to improve the reactivity of our optimizing DBT flow. Our results [44] show that, compared to a software implementation, our approach offers speed-up by 8$\times$ while consuming 18$\times$ less energy.

Our current research work investigates how DBT techniques can be used to support runtime configurable VLIW cores. Such cores enable fine grain exploration of energy/performance trade-off by dynamically adjusting their number of execution slots, their register file size, etc.). More precisely, we build on our DBT framework to enable dynamic code specialization. Our first experimental results suggest that this approach leads to best-case performance and energy efficiency when compared against static VLIW configurations [54].

Design Space Exploration for Iterative Stencil computations on FPGA accelerators

Participants : Steven Derrien, Gaël Deest, Tomofumi Yuki.

Iterative stencil computations arise in many application domains, ranging from medical imaging to numerical simulation. Since they are computationally demanding, a large body of work addressed the problem of parallelizing and optimizing stencils for multi-cores, GPUs, and FPGAs. Earlier attempts targeting FPGAs showed that the performance of such accelerators is the result of a complex interplay between the FPGA's raw computing power, the amount of on-chip memory it has, and the performance of the external memory system. They also illustrate how each application may have different requirements. For example, in the context of embedded vision, the designer's goal is often to find the design with minimum cost that matches real-time performance constraints (e.g., 4K@60fps). In an exascale context, the designer's goal is to maximize performance (measured in ops-per-second) for a given FPGA board, while maintaining power dissipation to a minimum. Based on these observations, we explore a family of design options that can accommodate a large set of requirements and constraints, by exposing trade-offs between computing power, bandwidth requirements, and FPGA resource usage. We have developed a code generator that produces HLS-optimized C/C++ descriptions of accelerator instances targeting emerging System on Chip platforms, (e.g., Xilinx Zynq or Intel SoC). Our family of designs builds upon the well-known tiling transformation, which we use to balance on-chip memory cost and off-chip bandwidth. To ease the exploration of this design space, we propose performance models to hone in on the most interesting design points, and show how they accurately lead to optimal designs. Our results demonstrate that the optimal choice depends on problem sizes and performance goals [30].

Energy-driven Accelerator Exploration for Heterogeneous Multiprocessor Architectures

Participants : Baptiste Roux, Olivier Sentieys.

Programming heterogeneous multiprocessor architectures combining multiple processor cores and hardware accelerators is a real challenge. Computer-aided design and development tools try to reduce the large design space by simplifying hardware software mapping mechanisms. However, energy consumption is not well supported in most of design space exploration methodologies due to the difficulty to fast and accurately estimate energy consumption. To this aim,we proposed and validated an exploration method for partitioning applications on software cores and hardware accelerators under energy-efficiency constraints. The methodology is based on energy and performance measurement of a tiny subset of the design space and an analytical formulation of the performance and energy of an application kernel mapped on a heterogeneous architecture. This closed-form expression is captured and solved using Mixed Integer Linear Programming, which allows for very fast exploration resulting in the optimal solution. The approach is validated on two applications kernels using Zynq-based architecture showing more than 12% acceleration speed-up and energy saving compared to standard approaches. Results also show that the most energy-efficient solution is application- and platform-dependent and moreover hardly predictable, which highlights the need for fast exploration.