Section: New Results

Reconfigurable Architecture and Hardware Accelerator Design

Algorithmic Fault Tolerance for Timing Speculative Hardware

Participants : Thibaut Marty, Tomofumi Yuki, Steven Derrien.

We have been working on timing speculation, also known as overclocking, to increase the computational throughput of accelerators. However, aggressive overclocking introduces timing errors, which may corrupt the outputs to unacceptable levels. It is extremely challenging to ensure that no timing errors occur, since the probability of such errors happening depends on many factors including the temperature and process variation. Thus, aggressive timing speculation must be coupled with a mechanism to verify that the outputs are correctly computed. Our previous result demonstrated that the use of inexpensive checks based on algebraic properties of the computation can drastically reduce the cost of verifying that overclocking did not produce incorrect outputs. This has allowed the accelerator to significantly boost its throughput with little area overhead.

One weakness coming from the use of algebraic properties is that the inexpensive check is not strictly compatible with floating-point arithmetic that is not associative. This was not an issue with our previous work that targeted convolutional neural networks, which typically use fixed-point (integer) arithmetic. Our on-going work aims to extend our approach to floating-point arithmetic by using extended precision to store intermediate results, known as Kulisch accumulators. At first glance, use of extended precision that covers the full exponent range of floating-point may look costly. However, the design space of FPGAs is complex with many different trade-offs, making the optimal design highly context dependent. Our preliminary results indicate that the use of extended precision may not be any more costly than implementing the computation in floating point.

Adaptive Dynamic Compilation for Low-Power Embedded Systems

Participants : Steven Derrien, Simon Rokicki.

Previous works on Hybrid-DBT have demonstrated that using Dynamic Binary Translation, combined with low-power in-order architecture, enables an energy-efficient execution of compute-intensive kernels. In [33], we address one of the main performance limitations of Hybrid-DBT: the lack of speculative execution. We study how it is possible to use memory dependency speculation during the DBT process. Our approach enables fine-grained speculation optimizations thanks to a combination of hardware and software mechanisms. Our results show that our approach leads to a geo-mean speed-up of 10% at the price of a 7% area overhead. In [49], we summarize the current state of the Hybrid-DBT project and display our last results about the performance and the energy efficiency of the system. The experimental results presented here show that, for compute-intensive benchmarks, Hybrid-DBT can deliver the same performance level than a 3-issue OoO core, while consuming three times less energy. Finally, in [34], we investigate security issues caused by the use of speculation in DBT-based systems. We demonstrate that, even if those systems use in-order micro-architectures, the DBT layer optimizes binaries and speculates on the outcome of some branches, leading to security issues similar to the Spectre vulnerability. We demonstrate that both the NVidia Denver architecture and the Hybrid-DBT platform are subject to such vulnerability. However, we also demonstrate that those systems can easily be patched, as the DBT is done in software and has fine-grained control over the optimization process.

What You Simulate Is What You Synthesize: Designing a Processor Core from C++ Specifications

Participants : Simon Rokicki, Davide Pala, Joseph Paturel, Olivier Sentieys.

Designing the hardware of a processor core as well as its verification flow from a single high-level specification would provide great advantages in terms of productivity and maintainability. In [32] (a preliminary version also in [42]), we highlight the gain of starting from a unique high-level synthesis and simulation C++ model to design a processor core implementing the RISC-V Instruction Set Architecture (ISA). The specification code is used to generate both the hardware target design through High-Level Synthesis as well as a fast and cycle-accurate bit-accurate simulator of the latter through software compilation. The object oriented nature of C++ greatly improves the readability and flexibility of the design description compared to classical HDL-based implementations. Therefore, the processor model can easily be modified, expanded and verified using standard software development methodologies. The main challenge is to deal with C++ based synthesizable specifications of core and uncore components, cache memory hierarchy, and synchronization. In particular, the research question is how to specify such parallel computing pipelines with high-level synthesis technology and to demonstrate that there is a potential high gain in design time without jeopardizing performance and cost. Our experiments demonstrate that the core frequency and area of the generated hardware are comparable to existing RTL implementations.

Accelerating Itemset Sampling on FPGA

Participants : Mael Gueguen, Olivier Sentieys.

Finding recurrent patterns within a data stream is important for fields as diverse as cybersecurity or e-commerce. This requires to use pattern mining techniques. However, pattern mining suffers from two issues. The first one, known as ”pattern explosion”, comes from the large combinatorial space explored and is the result of too many patterns outputted to be analyzed. Recent techniques called output space sampling solve this problem by outputting only a sampled set of all the results, with a target size provided by the user. The second issue is that most algorithms are designed to operate on static datasets or low throughput streams. In [24], we propose a contribution to tackle both issues, by designing an FPGA accelerator for pattern mining with output space sampling. We show that our accelerator can outperform a state-of-the-art implementation on a server class CPU using a modest FPGA product. This work is done in collaboration with A. Termier from the Lacodam team at Inria.

Hardware Accelerated Simulation of Heterogeneous Platforms

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

When considering designing heterogeneous multicore 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 multicore architectures. These limitations motivate the use of hardware to accelerate the simulation of heterogeneous multicore, and in particular of FPGA components. We study an approach for designing such systems based on performance models through combining accelerator and processor core models. These models are implemented in the HAsim/LEAP infrastructure. In [22], we propose a methodology for building performance models of accelerators and describe the defined design flow.

Fault-Tolerant Scheduling onto Multicore embedded Systems

Participants : Emmanuel Casseau, Minyu Cui, Petr Dobias, Lei Mo, Angeliki Kritikakou.

Demand on multiprocessor systems for high performance and low energy consumption still increases in order to satisfy our requirements to perform more and more complex computations. Moreover, the transistor size gets smaller and their operating voltage is lower, which goes hand in glove with higher susceptibility to system failure. In order to ensure system functionality, it is necessary to conceive fault-tolerant systems. Temporal and/or spatial redundancy is currently used to tackle this issue. Actually, multiprocessor platforms can be less vulnerable when one processor is faulty because other processors can take over its scheduled tasks. In this context, we investigate how to map and schedule tasks onto homogeneous faulty processors.

We consider two approaches. The first approach deals with task mapping onto processors at compile time. Our goal is to guarantee both reliability and hard real-time constraints with low-energy consumption. Task duplication is assessed and duplication is performed if expected reliability of a task is not met. This work concurrently decides duplication of tasks, the task execution frequency and task allocation to minimize the energy consumption of a multicore platform with Dynamic Voltage and Frequency Scaling (DVFS) capabilities. The problem is initially formulated as Integer Non-Linear Programming and equivalently transformed to a Mixed Integer Linear Programming problem to be optimally solved. The proposed approach provides a good trade-off between energy consumption and reliability. The second approach deals with mapping and scheduling tasks at runtime. The application context is CubeSats. CubeSats operate in harsh space environment and they are exposed to charged particles and radiations, which cause transient faults. To make CubeSats fault tolerant, we propose to take advantage of their multicore architecture. We propose two online algorithms, which schedule all tasks on board of a CubeSat, detect faults and take appropriate measures (based on task replication) in order to deliver correct results. The first algorithm considers all tasks as aperiodic tasks and the second one treats them as aperiodic or periodic tasks. Their performances vary, particularly when the number of processors is low, and a choice is subject to a trade-off between the rejection rate and the energy consumption. This work is done in collaboration with Oliver Sinnen, PARC Lab., the University of Auckland.

Run-Time Management on Multicore Platforms

Participant : Angeliki Kritikakou.

In time-critical systems, run-time adaptation is required to improve the performance of time-triggered execution, derived based on Worst-Case Execution Time (WCET) of tasks. By improving performance, the systems can provide higher Quality-of-Service, in safety-critical systems, or execute other best-effort applications, in mixed-critical systems. To achieve this goal, we propose a parallel interference-sensitive run-time adaptation mechanism that enables a fine-grained synchronisation among cores [37]. Since the run-time adaptation of offline solutions can potentially violate the timing guarantees, we present the Response-Time Analysis (RTA) of the proposed mechanism showing that the system execution is free of timing-anomalies. The RTA takes into account the timing behavior of the proposed mechanism and its associated WCET. To support our contribution, we evaluate the behavior and the scalability of the proposed approach for different application types and execution configurations on the 8-core Texas Instruments TMS320C6678 platform. The obtained results show significant performance improvement compared to state-of-the-art centralized approaches.

Energy Constrained and Real-Time Scheduling and Assignment on Multicores

Participants : Olivier Sentieys, Angeliki Kritikakou, Lei Mo.

Asymmetric Multicore Processors (AMP) are a very promising architecture to deal efficiently with the wide diversity of applications. In real-time application domains, in-time approximated results are preferred to accurate – but too late – results. In [28], we propose a deployment approach that exploits the heterogeneity provided by AMP architectures and the approximation tolerance provided by the applications, so as to increase as much as possible the quality of the results under given energy and timing constraints. Initially, an optimal approach is proposed based on the problem linearization and decomposition. Then, a heuristic approach is developed based on iteration relaxation of the optimal version. The obtained results show 16.3% reduction in the computation time for the optimal approach compared to conventional optimal approaches. The proposed heuristic approach is about 100 times faster at the cost of a 29.8% QoS degradation in comparison with the optimal solution.

Real-Time Energy-Constrained Scheduling in Wireless Sensor and Actuator Networks

Participants : Angeliki Kritikakou, Lei Mo.

Cyber-Physical Systems (CPS), as a particular case of distributed systems, raise new challenges, because of the heterogeneity and other properties traditionally associated with Wireless Sensor and Actuator Networks (WSAN), including shared sensing, acting and real-time computing. In CPS, mobile actuators can enhance system’s flexibility and scalability, but at the same time incur complex couplings in the scheduling and controlling of the actuators. In [19], we propose a novel event-driven method aiming at satisfying a required level of control accuracy and saving energy consumption of the actuators, while guaranteeing a bounded action delay. We formulate a joint-design problem of both actuator scheduling and output control. To solve this problem, we propose a two-step optimization method. In the first step, the problem of actuator scheduling and action time allocation is decomposed into two subproblems. They are solved iteratively by utilizing the solution of one in the other. The convergence of this iterative algorithm is proved. In the second step, an on-line method is proposed to estimate the error and adjust the outputs of the actuators accordingly. Through simulations and experiments,we demonstrate the effectiveness of the proposed method. In addition, many of the real-time tasks of CPS can be executed in an imprecise way. Such systems accept an approximate result as long as the baseline Quality-of-Service (QoS) is satisfied and they can execute more computations to yield better results, if more system resources are available. These systems are typically considered under the Imprecise Computation (IC) model, achieving a better tradeoff between QoS and limited system resources. However, determining a QoS-aware mapping of these real-time IC-tasks onto the nodes of a CPS creates a set of interesting problems. In [18], we firstly propose a mathematical model to capture the dependency, energy and real-time constraints of IC-tasks, as well as the sensing, acting, and routing in the CPS. The problem is formulated as a Mixed-Integer Non-Linear Programming (MINLP) due to the complex nature of the problem. Secondly, to efficiently solve this problem, we provide a linearization method that results in a Mixed-Integer Linear Programming (MILP) formulation of our original problem. Finally, we decompose the transformed problem into a task allocation subproblem and a task adjustment subproblem, and, then, we find the optimal solution based on subproblem iteration. Through the simulations, we demonstrate the effectiveness of the proposed method. Last, but not least, wireless charging can provide dynamic power supply for CPS. Such systems are typically considered under the scenario of Wireless Rechargeable Sensor Networks (WRSNs). With the use of Mobile Chargers (MCs), the flexibility of WRSNs is further enhanced. However, the use of MCs poses several challenges during the system design. The coordination process has to simultaneously optimize the scheduling, the moving time and the charging time of multiple MCs, under limited system resources (e.g., time and energy). Efficient methods that jointly solve these challenges are generally lacking in the literature. In [17], we address the multiple MCs coordination problem under multiple system requirements. Firstly, we aim at minimizing the energy consumption of MCs, guaranteeing that every sensor will not run out of energy. We formulate the multiple MCs coordination problem as a mixed-integer linear programming and derive a set of desired network properties. Secondly, we propose a novel decomposition method to optimally solve the problem, as well as to reduce the computation time. Our approach divides the problem into a subproblem for the MC scheduling and a subproblem for the MC moving time and charging time, and solves them iteratively by utilizing the solution of one into the other. The convergence of the proposed method is analyzed theoretically. Simulation results demonstrate the effectiveness and scalability of the proposed method in terms of solution quality and computation time.

Fault-Tolerant Microarchitectures

Participants : Joseph Paturel, Angeliki Kritikakou, Olivier Sentieys.

As transistors scale down, processors are more vulnerable to radiation that can cause multiple transient faults in function units. Rather than excluding these units from execution, performance overhead of VLIW processors can be reduced when fault-free components of these affected units are still used. In [30], the function units are enhanced with coarse-grained fault detectors. A re-scheduling of the instructions is performed at run-time to use not only the healthy function units, but also the fault-free components of the faulty function units. The scheduling window of the proposed mechanism covers two instruction bundles, which makes it suitable to explore mitigation solutions in the current and in the next instruction execution. Experiments show that the proposed approach can mitigate a large number of faults with low performance and area overheads. In addition, technology scaling can cause transient faults with long duration. In this case, the affected function unit is usually considered as faulty and is not further used. To reduce this performance degradation, we proposed a hardware mechanism to (i) detect the faults that are still active during execution and (ii) re-schedule the instructions to use the fault-free components of the affected function units [31]. When the fault faints, the affected function unit components can be reused. The scheduling window of the proposed mechanism is two instruction bundles being able to exploit function units of both the current and the next instruction execution. The results show multiple long-duration fault mitigation can be achieved with low performance, area, and power overhead.

Simulation-based fault injection is commonly used to estimate system vulnerability. Existing approaches either partially model the studied system’s fault masking capabilities, losing accuracy, or require prohibitive estimation times. Our work proposes a vulnerability analysis approach that combines gate-level fault injection with microarchitecture-level Cycle-Accurate and Bit-Accurate simulation, achieving low estimation time. Faults both in sequential and combinational logic are considered and fault masking is modeled at gate-level, microarchitecture-level and application-level, maintaining accuracy. Our case-study is a RISC-V processor. Obtained results show a more than 8% reduction in masked errors, increasing more than 55% system failures compared to standard fault injection approaches. This work is currently under review.

Fault-Tolerant Networks-on-Chip

Participants : Romain Mercier, Cédric Killian, Angeliki Kritikakou, Daniel Chillet.

Network-on-Chip has become the main interconnect in the multicore/manycore era since the beginning of this decade. However, these systems become more sensitive to faults due to transistor shrinking size. In parallel, approximate computing appears as a new computation model for applications since several years. The main characteristic of these applications is to support the approximation of data, both for computations and for communications. To exploit this specific application property, we develop a fault-tolerant NoC to reduce the impact of faults on the data communications. To address this problem, we consider multiple permanent faults on router which cannot be managed by Error-Correcting Codes (ECCs) and we propose a bit-shuffling method to reduce the impact of faults on Most Significant Bits (MSBs), hence permanent faults only impact Low Significant Bits (LSBs) instead of MSBs reducing the errors impact. We evaluated the proposed method for data mining benchmark and we show that our proposal can lead to 73.04% reduction on the clustering error rate and 84.64% reduction on the mean centroid Mean Square Error (MSE) for 3-bit permanent faults which affect MSBs on 32-bit words with a limited area cost. This work is currently under review for an international conference.

Improving the Reliability of Wireless Network-on-Chip (WiNoC)

Participants : Joel Ortiz Sosa, Olivier Sentieys, Cédric Killian.

Wireless Network-on-Chip (WiNoC) is one of the most promising solutions to overcome multi-hop latency and high power consumption of modern many/multi core System-on-Chip (SoC). However, standard WiNoC approaches are vulnerable to multi-path interference introduced by on-chip physical structures. To overcome such parasitic phenomenon, we first proposed a Time-Diversity Scheme (TDS) to enhance the reliability of on-chip wireless links using a realistic wireless channel model. We then proposed an adaptive digital transceiver, which enhances communication reliability under different wireless channel configurations in [39]. Based on the same realistic channel model, we investigated the impact of using some channel correction techniques. Experimental results show that our approach significantly improves Bit Error Rate (BER) under different wireless channel configurations. Moreover, our transceiver is designed to be adaptive, which allows for wireless communication links to be established in conditions where this would not be possible for standard transceiver architectures. The proposed architecture, designed using a 28-nm FDSOI technology, consumes only 3.27 mW for a data rate of 10 Gbit/s and has a very small area footprint. We also proposed a low-power, high-speed, multi-carrier reconfigurable transceiver based on Frequency Division Multiplexing (FDM) to ensure data transfer in future Wireless NoCs in [38]. The proposed transceiver supports a medium access control method to sustain unicast, broadcast and multicast communication patterns, providing dynamic data exchange among wireless nodes. Designed using a 28-nm FDSOI technology, the transceiver only consumes 2.37 mW and 4.82 mW in unicast/broadcast and multicast modes, respectively, with an area footprint of 0.0138 mm${}^2$.

Error Mitigation in Nanophotonic Interconnect

Participants : Jaechul Lee, Cédric Killian, Daniel Chillet.

The energy consumption of manycore is dominated by data movements, which calls for energy-efficient and high-bandwidth interconnects. Integrated optics is promising technology to overcome the bandwidth limitations of electrical interconnects. However, it suffers from high power overhead related to low efficiency lasers, which calls for the use of approximate communications for error tolerant applications. In this context, in [26] we investigate the design of an Optical NoC supporting the transmission of approximate data. For this purpose, the least significant bits of floating point numbers are transmitted with low power optical signals. A transmission model allows estimating the laser power according to the targeted BER and a micro-architecture allows configuring, at run-time, the number of approximated bits and the laser output powers. Simulation results show that, compared to an interconnect involving only robust communications, approximations in the optical transmissions lead to a laser power reduction up to 42% for image processing application with a limited degradation at the application level.