Full-system Simulation Research Articles

Backbone curve tailoring via Lyapunov subcenter manifold optimization

We present a technique for the direct optimization of conservative backbone curves in nonlinear mechanical systems. The periodic orbits on the conservative backbone are computed analytically using the reduced dynamics of the corresponding Lyapunov subcenter manifold (LSM). In this manner, we avoid expensive full-system simulations and numerical continuation to approximate the nonlinear response. Our method aims at tailoring the shape of the backbone curve using a gradient-based optimization with respect to the system’s parameters. To this end, we formulate the optimization problem by imposing constraints on the frequency-amplitude relation. Sensitivities are computed analytically by differentiating the backbone expression and the corresponding LSM. At each iteration, only the reduced-order model construction and sensitivity computation are performed, making our approach robust and efficient.

Expanding hardware accelerator system design space exploration with gem5-SALAMv2

With the prevalence of hardware accelerators as an integral part of the modern systems on chip (SoCs), the ability to model accelerators quickly and accurately within the system in which it operates is critical. This paper presents gem5-SALAMv2 as a novel system architecture for LLVM-based modeling and simulation of custom hardware accelerators integrated into the gem5 framework. It overcomes the inherent limitations of state-of-the-art trace-based pre-register-transfer level (RTL) simulators by offering a truly “execute-in-execute” LLVM-based model. It enables scalable modeling of multiple dynamically interacting accelerators with full-system simulation support. To create long-term sustainable expansion compatible with the gem5 system framework, gem5-SALAM offers a general-purpose and modular communication interface and memory hierarchy integrated into the gem5 ecosystem, streamlining designing and modeling accelerators for new and emerging applications. gem5-SALAMv2 expands upon the framework established in gem5-SALAMv1 with improved LLVM-based elaboration and simulation, improved and more extensible system integration, and new automations to simplify rapid prototyping and design space exploration. 11Conference Paper Extension: This work extends the work presented in gem5-SALAM: A System Architecture for LLVM-based Accelerator Modeling from MICRO 2020 (Rogers et al., 2020). This work expands on the aforementioned work by revamping the gem5-SALAM internals to provide more robust and extensible simulations, introducing new automation tools for expanding and simplifying design space exploration, and demonstrates the new capabilities of gem5-SALAMv2 by exploring multiple configurations of simple neural network architectures.Validation on the MachSuite (Reagen et al., 2014) benchmarks presents a timing estimation error of less than 1% against the Vivado High-Level Synthesis (HLS) tool. Results also show less than a 4% area and power estimation error against Synopsys Design Compiler. Additionally, system validation against implementations on an Ultrascale+ ZCU102 shows an average end-to-end timing error of less than 2%. Lastly, we demonstrate the upgraded capabilities of gem5-SALAMv2 by exploring accelerator platforms for two deep neural networks, LeNet5 and MobileNetv2. In these explorations, we demonstrate how gem5-SALAMv2 can simulate such systems and guide architectural optimizations for these types of accelerator-rich architectures. 22The most up-to-date version of gem5-SALAMv2 is available at https://github.com/TeCSAR-UNCC/gem5-SALAM..

RSPP: Restricted Static Pseudo-Partitioning for Mitigation of Cross-Core Covert Channel Attacks

Cache timing channel attacks exploit the inherent properties of cache memories: hit and miss time along with the shared nature of the cache to leak secret information. The side channel and covert channel are the two well-known cache timing channel attacks. In this article, we propose Restricted Static Pseudo-Partitioning (RSPP), an effective partition-based mitigation mechanism that restricts the cache access of only the adversaries involved in the attack. It has an insignificant impact of only 1% in performance, as the benign processes have access to the full cache and restrictions are limited only to the suspicious processes and cache sets. It can be implemented with a maximum storage overhead of 1.45% of the total Last-Level Cache (LLC) size. This article presents three variations of the proposed attack mitigation mechanism: RSPP, simplified-RSPP (S-RSPP) and corewise-RSPP (C-RSPP) with different hardware overheads. A full system simulator is used for evaluating the performance impact of RSPP. A detailed experimental analysis with different LLC and attack parameters is also discussed. RSPP is also compared with the existing defense mechanisms effective against cross-core covert channel attacks.

Data-driven dynamical coarse-graining for condensed matter systems.

Simulations of condensed matter systems often focus on the dynamics of a few distinguished components but require integrating the full system. A prime example is a molecular dynamics simulation of a (macro)molecule in a solution, where the molecule(s) and the solvent dynamics need to be integrated, rendering the simulations computationally costly and often unfeasible for physically/biologically relevant time scales. Standard coarse graining approaches can reproduce equilibrium distributions and structural features but do not properly include the dynamics. In this work, we develop a general data-driven coarse-graining methodology inspired by the Mori-Zwanzig formalism, which shows that macroscopic systems with a large number of degrees of freedom can be described by a few relevant variables and additional noise and memory terms. Our coarse-graining method consists of numerical integrators for the distinguished components, where the noise and interaction terms with other system components are substituted by a random variable sampled from a data-driven model. The model is parameterized using data from multiple short-time full-system simulations, and then, it is used to run long-time simulations. Applying our methodology to three systems-a distinguished particle under a harmonic and a bistable potential and a dimer with two metastable configurations-the resulting coarse-grained models are capable of reproducing not only the equilibrium distributions but also the dynamic behavior due to temporal correlations and memory effects. Remarkably, our method even reproduces the transition dynamics between metastable states, which is challenging to capture correctly. Our approach is not constrained to specific dynamics and can be extended to systems beyond Langevin dynamics, and, in principle, even to non-equilibrium dynamics.

NDPmulator: Enabling Full-System Simulation for Near-Data Accelerators From Caches to DRAM

NDPmulator: Enabling Full-System Simulation for Near-Data Accelerators From Caches to DRAM

Nitrogen offset potential in a multiyear farmlet-scale study: Milk and herbage production from grazed perennial ryegrass–white clover swards

The objective of this study was to quantify the farm gate nitrogen (N) offset potential of perennial ryegrass (Lolium perenne L.; PRG) white clover (Trifolium repens L.; WC) swards by comparing the herbage and milk production from dairy farmlets that were simulations of full farming systems. A study was established where 120 cows were randomly assigned to 4 farmlets of 10.9 ha (stocking rate: 2.75 cow/ha), composed of 20 paddocks each. Cows were fed 526 kg of DM of concentrate on average each year. The 4 grazing treatments were PRG-only at 150 or 250 kg of N/ha and PRG-WC at 150 or 250 kg of N/ha. Cows remained in their treatment group for an entire grazing season and were re-randomized as they calved across treatments each year. As cows calved in the spring as standard practice in Ireland, they were rotationally grazed from early February both day and night (weather permitting) to mid-November, to a target postgrazing sward height of 4.0 cm. Mean sward WC content was 18.1% and 15.4% for the 150 and 250 kg of N/ha PRG-WC treatments, respectively over the 3-yr period. When WC was included, lowering the N rate did not reduce pregrazing yield, pregrazing height, or herbage removed, but those factors decreased significantly when WC was absent. Total annual herbage DM production was 13,771, 15,242, 14,721, and 15,667 kg of DM/ha for PRG-only swards receiving 150 or 250 kg of N/ha and PRG-WC swards receiving 150 or 250 kg of N/ha, respectively. In addition, when WC was present, compressed postgrazing sward heights were lower (4.10 vs. 4.21 cm) and herbage allowance (approximately 17 kg/cow feed allocation per cow per day) higher than the high-N control (+ 0.7 kg of DM/cow per day). There was a significant increase in milk production, both per cow and per hectare, when WC was included in PRG swards. Over the 3-yr study, cows grazing PRG-WC had greater milk (+304 kg) and milk solids (+31 kg of fat + protein) yields than cows grazing PRG-only swards. This significant increase in milk production suggests that the inclusion of WC in grazing systems can be effectively used to increase milk production per cow and per hectare and help offset nitrogen use. This result shows the potential to increase farm gate N use efficiency and reduce the N surplus compared with PRG-dominant sward grazing systems receiving 250 kg of N/ha, without negatively affecting milk solids yield or herbage production, thus increasing farm profit by €478/ha.

Operand-Oriented Virtual Memory Support for Near-Memory Processing

Virtual memory support is one of the major challenges of near-memory processing (NMP). Many previous works focused on this issue, but there are practical limitations that conventional CPU hardware or memory allocation schemes should be modified. Another technique uses a specialized page table for NMP to avoid such limitations. However, the previous work proposed NMP-specific page table that has static page table walk latency regardless of data size. This causes unnecessarily long address translation time for relatively small data. In this paper, we propose an operand-oriented technique for virtual memory support. Our scheme does not pre-determine the size of shared space; rather, it allocates shared space depending on the size of operands data for NMP. Then, we significantly reduce page table walk latency by using our flexible page table, which adapts the page table hierarchy to the size of shared spaces. To prove our concept, we implement our scheme in a full-system simulator and an FPGA-based verification platform. We then compared it with CPU's page table and the previous NMP-specific page table. The experimental results show that our technique outperforms page table walk latency by 69.3 percent and 43.8 percent compared to the CPU's page table and the comparison, respectively.

Toward Software-like Debugging for FPGAs via Checkpointing and Transaction-based Co-Simulation

Checkpoint-based debugging flows have recently been developed that allow the user to move the design state back and forth between an FPGA and a simulator. They provide a softwarelike debugging experience by combining the speed of hardware execution and the full visibility of simulation. However, they assume the entire system state can be moved to a simulator, limiting them to self-contained systems. In this article, we present StateLink, a transaction-based co-simulation framework that allows part of the system (the task) to run in a simulator and still interact with other system components that reside in hardware. StateLink allows tasks to remain connected to and active in the overall hardware system after their state is moved to a simulator. This extends the functionality of checkpoint-based debugging frameworks to designs with external I/Os and significantly speeds up the simulation of tasks that are part of a large system. StateLink typically adds no timing overhead and a modest hardware area overhead. The total area overhead of using the proposed flow on a Memcached system is only 13%. This flow allows the user to benefit from both the hardware speedup of ∼1M× and the StateLink speedup of up to 44× versus full system simulation.

Investigation of hybrid power plant configurations for an offshore vessel with co-simulation approach

This work presents a full-system simulator consisting of a generic power system integrated with a vessel model and real-time capabilities. The whole system simulation facilitates evaluating the overall system performance by considering components’ interactions according to the maneuvering and environmental effects. Indeed, flexibility in the configuration and size of the power system enables the investigation of different concepts according to various maneuvering scenarios. Co-simulation approach is employed to integrate the models with various domains effectively. In addition, the bond graph modeling strategy as a power based method is used. The developed power system contains a diesel genset, a PEMFC, a battery with average electrical components, and a power management system. The configuration of the power system and size of power sources are modifiable. Various hybrid configurations and power capacities can be designed with validated power sources against marine vendors. In addition, the integrated offshore supply vessel with Dynamic Positioning (DP) and cruise controller and sea state forces induces the corresponding load demand of the operation to the power system. In summary, different operation scenarios with sea states, the thrusters’ states and allocation algorithm, DC link voltage, power electrical converters controller, and fuel consummations are captured in one model framework. To demonstrate the application of the model and emphasis the importance of total system simulation, three power system configurations are designed and simulated with two operational modes of DP and cruise with various sea states.

Performance Tuning via Lean Measurements for Acceleration of Network Functions Virtualization

Network Functions Virtualization (NFV) replaces the specialized hardware with the software-based forwarding to promise the flexibility, scalability and automation benefits. With an increasing range of applications, NFV must ultimately forward packets at rates that are comparable to the native and specialized hardware-based approaches. However, the transition packet forwarding from specialized hardware to software-based has turned out to be more challenging than expected. Thus, NFV acceleration is desperately needed to play a crucial role in the development of NFV. It is an interesting issue how to address the persistent performance tuning in a way that provides far greater flexibility to meet the demands of power. The existing developments are very inefficient, since that the uncontrollable and unanticipated performance regressions frequently occur. Besides, the environments for full system simulations are traditionally expensive and time consuming to evaluate the system performance. In this paper, we propose the methodology named as “NFV Acceleration via Lean Measurements (NALM)” to tune the performance for the NFV acceleration. NALM provides a holistic measurement approach through combining individual measures to quickly identify the bottlenecks, which can help developers with a better understanding of the design tradeoffs. Moreover, the environments for large scale performance simulation are replaced by a debugger. Thus, the waste is eliminated in terms of time consumption and infrastructure costs of the full system simulation. The systematic analysis of the multi-cores speedup ratio highlights the potential optimization space and rules. We further propose the improvement recommendations on efficient practices. The experiments evaluate the specific effects, and the relationship between the metrics and forwarding performance.

CAPMIG: Coherence-Aware Block Placement and Migration in Multiretention STT-RAM Caches

In recent years, the increased working set size of applications craves more memory demand in terms of large-sized last-level caches (LLCs). To fulfill this, one of the promising technology is STTRAM. However, high write energy and write latency make it challenging to adopt it on a wide scale. Multiretention STTRAM caches have been considered an improvisation over standard STTRAM caches by reducing their retention time which reduces the write latency. Here, we have to negotiate with a refresh operation by applying various refresh management techniques. However, its retention period consumes significant refresh energy in the periodic refresh. In this article, we take help from the coherence protocol and decide the best retention type for each block. A block loaded on a write access is likely to get more writes in the future and is therefore loaded in the lowest retention time region. Similarly, instruction blocks are loaded in the highest retention time region. This helps in reducing the number of refreshes incurred by the blocks. During runtime, the blocks may change their access patterns, requiring a change in their retention region. This article also proposes a migration policy to relocate the blocks to appropriate regions during runtime. Identification of zero data value blocks and not refreshing them is an additional augmentation to our proposal. The experimental result using full system simulation shows a good reduction in the number of refreshes and energy consumption over the baseline designs.

Assessment of CO2 enrichment mechanism in integrated coal gasification fuel cell combined cycle system with carbon capture

The present research proposes an innovative multi-physics coupled model of different configurations of an integrated coal gasification fuel cell combined cycle (IGFC) system employing Solid Oxide Electrolytic Cell (SOEC) for CO2 capture. Full-system simulation is carried out to examine efficiency. The model incorporates a Solid Oxide Fuel Cell (SOFC), a SOEC, a gas turbine (GT), and multiple recirculation loops operated by two ejectors. The results reveal that compared with traditional power plants, the proposed IGFC system equipped with SOEC can reduce CO2 emission by almost 80%, and operates environmentally beneficial. The efficiency of the system varies greatly depending on the design parameters implemented. The CO2 enrichment phenomenon by SOFC and capture measures of CO2 by SOEC are simultaneously analyzed. In addition, parametric analysis is performed to evaluate the coupling influence of multiple operating parameters on the IGFC system. Recirculation ratios of 0.75 with four times recirculations are found to be the optimal conditions for both SOFC fuel electrode and SOEC air electrode aimed at getting to the highest power generation efficiency and total CO2 capture rate of the system. After systematic optimization of the design parameters, the electrical efficiency and CO2 capture rate of the proposed system could achieve 68.47% and 87.88%, respectively, which are about 20% and 60% greater than those of traditional power plants. Furthermore, after optimizing the control strategy, the fuel utilization rate of the system increases from 63.09% to 83.40%.

PV Solar Yield Applicable Simulator Based on Free Irradiance Data Source: Applied Comprehensive Tool for Solar Engineers

Accurate PV system simulators are implemented with expensive software platforms using paid irradiance data. The main purpose of this paper is to develop and validate a PV system simulator, beginning with a solar cell parameter extraction model, then test and validate long-term Irradiance data using free online source (Typical Meteorological Year TMY in (PVGIS) European website), and finally building full solar generator simulator to run in working real conditions. Comparing results with Accurate Paid PV simulators (which use the Muneer model) showed good accuracy of the proposed simulator. Work flow starts with the Irradiance model’s data processing, then solar cell 5 parameters model data processing (to extract cell parameters), and finally full system simulator. MATLAB coding programs in real working conditions are used for simulation. Results of solar cell parameter extraction show 99.6% to 99.99% matching with data sheet and cell performance under standard test conditions. System model simulation output shows 8% less yearly generated energy compared to the PVGIS 2022 long-term simulation (hourly basis (one-year time)). This is due to incident energy variations (between the years 2016 and 2022) of 4.02%. The novelty of the algorithm is the methodology, as it tests irradiance data on an hourly basis and validates results for a whole continuous year. Also, the 5-parameter solar cell model is used to be validated in long term analysis, not only STC conditions and could be applied on any PV solar cell. The algorithm and block diagram used are scalable, modular, and interchangeable with similar models to be tested. This simulator could test several methods and models in solar pv technology.

NCDE: In-Network Caching for Directory Entries to Expedite Data Access in Tiled-Chip Multiprocessors

The processing of data-intensive applications, followed by an unprecedented amount of data traffic, drives explosive accesses to the memory subsystem. The overloaded memory subsystem experiences increased data access latency. To expedite data access, a network caching technique that leverages network-on-chip (NoC) virtual channels (VCs) as an expanded memory subsystem has emerged. Previous network caching studies focused on utilizing VCs on the NoC’s local input port as a victim cache to reduce local data access latency. In contrast to previous studies, we explore the opportunity of mitigating problems associated with shared data access via in-network caching for directory entries (NCDE), which can utilize every input port’s VCs to hold directory entries. NCDE exploits VCs as the victim and prefetch buffers of the directory entries, each reducing directory eviction-induced invalidations and simplifying the cache-to-cache (C2C) data transfer. The effectiveness of NCDE was evaluated using a gem5 full-system simulator, and the results show that the average memory access time (AMAT) and workload execution time were reduced by 7.69% and 5.82%, respectively. As a cost for accelerating the data access latency, implementing NCDE incurs a negligible router area overhead of 1.56%.

A complete compact model for simulation of organic electronic systems

Organic Electronics provides multiple technological advantages such as low cost, flexibility, solution processability and waste reduction through the use of new materials and alternative fabrication techniques in the microelectronic world. The use of additive and printed fabrication processes allows the development of instantly fabricated, fully additive, low temperature processing together with lightweight, bendable, scalable and sustainable systems. Different models have been presented in the field but none of them with an industrial implementation. Complete simulation models are very useful for circuit production and commercialization since the organic electronics market is expected to grow in the near future and the need for industrial solutions for designers will increase. For this reason and for the first time, we present a compact model of DC and AC behaviour combined with different ambient conditions and parameter variability for full organic electronic system simulations.

A high-performance, metallodielectric 2D photonic crystal for thermophotovoltaics

Fuel-combustion-based thermophotovoltaic (TPV) systems are emerging as a viable power source for small, portable generators for a spectrum of applications such as UAVs, robotic platforms, and sensors. In TPV systems, an emitter heated to above 1000 K emits radiation that is then converted to electricity by a low bandgap photovoltaic cell. One promising class of TPV emitters are two-dimensional photonic crystals (PhCs) made of tantalum, which have shown high-temperature stability at 1150–1250K over hundreds of hours [1] , [2] and have been implemented in a prototype system with 4.4% fuel-to-electricity efficiency [1] . Tantalum PhCs filled and capped with hafnium oxide can enable even higher optical performance with in-band emissivities of 0.8–0.9. However, two key features are difficult to realize simultaneously: a uniformly filled cavity and a thin capping layer of hafnium oxide [3] , [4] . Here, we present a process that results in reduced roughness and better thickness control of the capping layer. We use room temperature reflectance measurements and full system simulations to estimate system performance gains. This selective emitter paves the way toward efficient, practical, and portable mesoscale generators. • A broadband selective emitter for TPV has been fabricated and measured. • Emitter consists of tantalum photonic crystal, filled and capped with hafnium oxide. • Up to 37.5% increase in power output is estimated over previous system prototype.

Accelerating the simulation of annual bifacial illumination of real photovoltaic systems with ray tracing

SummaryAccurate modeling of bifacial illumination is critical to improve the prediction of the energy yield of bifacial solar systems. Monte Carlo ray tracing is the most powerful tool to accomplish this task. In this work, we accelerate Monte Carlo ray tracing of large solar systems by nearly 90%. Our model achieves root-mean-square error values of 7.9% and 37.2% for the front and rear irradiance compared against single-axis tracking field reference data, respectively. The rear irradiance modeling error decreases to 18.9% if suspected snow periods are excluded. Crucially, our full system simulations show that surrounding ground surfaces affect the rear irradiance deep into the system. Therefore, unit system simulations cannot necessarily ignore the influence of the perimeter of large installations to accurately estimate annual yield. Large-scale simulations involving high-performance supercomputing were necessary to investigate these effects accurately, calibrate our simplified models, and validate our results against experimental measurements.

Depletion dependency of molten salt reactor dynamics

Fast dynamic simulations of full reactor systems at reduced fidelity are essential for prototyping and improving these complex machines. This paper presents a publicly available nodal dynamic model of the Molten Salt Demonstration Reactor (MSDR) implemented in MATLAB Simulink. The presented model is an iteration of a previously-published MSDR nodal dynamic model; capabilities introduced into this version include dynamic decay heat production and removal along with a novel depletion accounting module that affords full life cycle simulations. Factorization of the transient progression time and a depletion time is a key feature of this method. The model is intended to be used as a first-order engineering estimation model and a demonstration for method full life cycle evaluation of such molten salt reactor systems. Several normal and off-normal transients are evaluated along the full life cycle and discussed. Parameter sensitivity methodology is extended to depletion dependency and demonstrated on two example sensitivity studies.

Scaling up electronic structure calculations on quantum computers: The frozen natural orbital based method of increments.

The method of increments and frozen natural orbital (MI-FNO) framework is introduced to help expedite the application of noisy, intermediate-scale quantum (NISQ) devices for quantum chemistry simulations. The MI-FNO framework provides a systematic reduction of the occupied and virtual orbital spaces for quantum chemistry simulations. The correlation energies of the resulting increments from the MI-FNO reduction can then be solved by various algorithms, including quantum algorithms such as the phase estimation algorithm and the variational quantum eigensolver (VQE). The unitary coupled-cluster singles and doubles VQE framework is used to obtain correlation energies for the case of small molecules (i.e., BeH2, CH4, NH3, H2O, and HF) using the cc-pVDZ basis set. The quantum resource requirements are estimated for a constrained geometry complex catalyst that is utilized in industrial settings for the polymerization of α-olefins. We show that the MI-FNO approach provides a significant reduction in the quantum bit (qubit) requirements relative to the full system simulations. We propose that the MI-FNO framework can create scalable examples of quantum chemistry problems that are appropriate for assessing the progress of NISQ devices.

PACT: An Extensible Parallel Thermal Simulator for Emerging Integration and Cooling Technologies

Thermal analysis is an essential step that enables co-design of the computing system (i.e., integrated circuits and computer architectures) with the cooling system (e.g., heat sink). Existing thermal simulation tools are limited by several major challenges that prevent them from providing fast solutions to large problem sizes that are necessary to conduct standard-cell level thermal analysis or to evaluate new technologies or large chips. To overcome these challenges, we introduce a SPICE-based parallel compact thermal simulator (PACT) that achieves fast and accurate, standard cell to architecture-level, steady-state, and transient parallel thermal simulations. PACT utilizes the advantages of multicore processing (OpenMPI) and includes several solvers to speed up both steady-state and transient simulations. PACT can be easily extended to model a variety of emerging integration and cooling technologies by simply modifying the thermal netlist. In addition, PACT can also be used with popular architecture-level performance and power simulators. In comparison to a state-of-the-art finite-element method (FEM)-based simulator (COMSOL), PACT has a maximum error of 2.77% and 3.28% for steady-state and transient thermal simulations, respectively. Compared to a popular compact thermal simulator, HotSpot, PACT demonstrates a speedup of up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.83\times $ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$186\times $ </tex-math></inline-formula> for steady-state and transient simulations, respectively. We also show the applicability and extensibility of PACT through modeling emerging integration and cooling technologies, such as monolithic 3-D integrated circuits and liquid cooling via microchannels, and full-system simulation integration on a 2.5-D system with silicon-photonic network-on-chips (PNoCs).