Frédéric Pétrot

COSY communication IP's

The Fsprit/OMI-COSY project defines transaction-levels to set-up the exchange of IPs in separating function from architecture and body-behavior from proprietary interfaces. These transaction-levels are supported by the “COSY COMMUNICATION IPs” that are presented in this paper. They implement onto Systems-On-Chip the extended Kahn Process Network that is defined in COSY for modeling signal processing applications. We present a generic implementation and performance model of these system-level communications and we illustrate specific implementations. They set system communications across software and hardware boundaries, and achieve bus independence through the Virtual Component Interface of the VSI Alliance. Finally, we describe the COSY-VCC flow that supports communication refinement from specification, to performance estimation, to implementation.

Programming models and HW-SW interfaces abstraction for multi-processor SoC

For the design of classic computers the parallel programming concept is used to abstract HW/SW interfaces during high level specification of application software. The software is then adapted to existing multiprocessor platforms using a low level software layer that implements the programming model. Unlike classic computers, the design of heterogeneous MPSoC includes also building the processors and other kind of hardware components required to execute the software. In this case, the programming model hides both hardware and software refinements. This paper deals with parallel programming models to abstract both hardware and software interfaces in the case of heterogeneous MPSoC design. Different abstraction levels are needed. For the long term, the use of higher level programming models open new vistas for optimization and architecture exploration like CPU/RTOS tradeoffs

Elevator-First: A Deadlock-Free Distributed Routing Algorithm for Vertically Partially Connected 3D-NoCs

In this paper, we propose a distributed routing algorithm for vertically partially connected regular 2D topologies of different shapes and sizes (e.g., 2D mesh, torus, ring). The topologies that are the target of this algorithm are of practical interest in the 3D integration of heterogeneous dies using Through-Silicon-Vias (TSVs). Indeed, TSV-based 3D integration allows to envision the stacking of dies with different functions and technologies, using as an interconnect backbone a 3D-NoC. Intrinsically, 3D topologies have better performances, but yield and active area (and thus the cost) are function of the number of TSVs; therefore, the designs tend to use only a subset of available TSVs between two dies. The definition of blockage free and low implementation cost distributed deterministic routing on this kind of topology is thus of theoretical and practical interests. We formally prove that independently of the shape and dimensions of the planar topologies and of the number and placement of the TSVs, the proposed routing algorithm using two virtual channels in the plane is deadlock and livelock free. We also experimentally show that the performance of this algorithm is still acceptable when the number of vertical connections decreases.

Using binary translation in event driven simulation for fast and flexible MPSoC simulation

In this paper, we investigate the use of instruction set simulators (ISS) based on binary translation to accelerate full timed multiprocessor system simulation at transaction level. To have an accurate timing behavior, we had to firstly solve timing issues in processor modeling, secondly define fast and precise cache models, and thirdly solve the synchronization issues due to the different models of computation used in the ISSes and in the rest of the system. We present an integration solution that covers these issues and detail its implementation. We have experimented our proposal using processors models provided by the QEMU framework to replace the existing ISSes and SystemC TLM as simulation environment for the whole platform. This approach proposes a range of solutions trading off simulation speed versus accuracy. The experiments show that even for the most precise configuration, the simulation speedup is still significant.

Platform-based design from parallel C specifications

This paper presents Disydent, a framework dedicated to system-on-a-chip (SoC) platform-based design for shared memory multiple instructions multiple data (MIMD) architectures. We define a platform-based design problem as a triplet (system, application, constraints) where the system is both an operating system (OS) and a hardware (HW) template that can be enhanced with dedicated coprocessors. Our contributions are: 1) the definition of a complete flow for platform-based design, from application to integration including all necessary intermediate steps and 2) a set of tightly bound operational tools to implement the flow. Disydent is based on four tools. The distributed process network (DPN) is a C library for describing Kahn process network (KPN)-based applications. The asynchronous serial interface mode register 0 (ASIM0) is a multiprocessor target platform running a microkernel. This platform can be enhanced with coprocessors generated by the user-guided high-level synthesis (UGH) tool. Cycle accurate system simulator (CASS) is a high-performance cycle-accurate simulator. The main steps of the design flow are KPN modeling, functional validation, design space exploration, high-level synthesis, and temporal validation. The design flow starts by modeling the application as a KPN. This initial description is done in C using the DPN library. The functional validation is performed by running the initial description directly on the host. Without modifying the initial description, the user can simulate a HW/software (SW) partitioning by indicating the number of processors and the processes that are to be migrated to HW. This simulation is done at the cycle-accurate level for the whole system, except for the migrated processes for which the user must provide estimated time models. The description of the processes that are selected for HW implementation must be translated into a subset of C and then synthesized. This new description is still compatible with the DPN library, so it can be used for functional validation. The temporal validation is done at the cycle-accurate level using the initial description for the SW processes and cycle-accurate models automatically generated from the C subset description for the HW processes. Disydents strength relies on its formal KPN model that ensures a behavior that is independent of the overall system scheduling, its fast cycle-accurate validation that is several orders of magnitude faster than classical event-driven simulators, and its single description of a process that is used as input of DPN, CASS, and UGH.

Automatic instrumentation of embedded software for high level hardware/software co-simulation

We propose an automatic instrumentation method for embedded software annotation to enable performance modeling in high level hardware/software co-simulation environments. The proposed “cross-annotation” technique consists of extending a retargetable compiler infrastructure to allow the automatic instrumentation of embedded software at the basic block level. Thus, target and annotated native binaries are guaranteed to have isomorphic control flow graphs (CFG). The proposed method takes into account the processor-specific optimizations at the compiler level and proves to be accurate with low simulation overhead.

Lightweight Implementation of the POSIX Threads API for an On-Chip MIPS Multiprocessor with VCI Interconnect

This paper relates our experience in designing from scratch a multi-threaded kernel for a MIPS R3000 on-chip multiprocessor. We briefly present the target architecture build around a VCI compliant interconnect, and the CPU characteristics. Then we focus on the implementation of part of the POSIX 1003.1b and 1003.1c standards. We conclude this case study by simulation results obtained by cycle true simulation of an MJPEG video decoder application on the multiprocessor using several scheduler organizations and architectural parameters.

On Cache Coherency and Memory Consistency Issues in NoC Based Shared Memory Multiprocessor SoC Architectures

The concept of network on chip (NoC) is a recent breakthrough in the system on chip (SoC) design area. A lot of work has been done to define efficient NoC architectures and implementations. In this paper, our goal is twofold. Firstly, we want to outline that the use of a NoC based shared-memory multiprocessor SoC challenges the application integrator because of the underlying assumptions of software, namely cache coherency and memory consistency. These problems are well known in general purpose shared memory multiprocessors. However, when designing a SoC, we benefit on the one hand from the knowledge of the applications, the much simpler usage of virtual memory, lower interconnect latencies and very high bandwidth at lost cost, but on the other hand we suffer from more tight design constraints (yield, power, predictable performances, ...). Secondly, we define simple and yet attractive solutions - in term of design time and hardware cost - to both problems in the context of application specific multiprocessor SoCs

A System Framework for the Design of Embedded Software Targeting Heterogeneous Multi-core SoCs

Embedded appliances designers rely on Heterogeneous Multi-Core System-on-Chips (HMC-SoC) to provide the computing power required by modern applications. Due to the inherent complexity of this kind of platform, the development of speci¿c system architectures is not considered as an option to provide low-level services to an application. Hence, the software is built either from scratch - when the software’s requirements are not too high - or over a general-purpose operating system, leading to performance and memory usage trade-offs. Our contribution is a component-based system framework that provides high-level system services for embedded software applications with few impacts on the memory usage and ¿nal performances, thanks to strong interfaces that enable the reuse of existing software elements and facilitate the support of multiple hardware platforms. The ef¿ciency of our approach is demonstrated on an existing MC-SoC.

Cycle precise core based hardware/software system simulation with predictable event propagation

We present a simple technique for efficient cycle precise core based system simulator implementation. We first examine the current communication mechanisms in state-of-the-art digital embedded systems, and notice that few signals depend on signals set during the same cycle. Using a system model based on communicating finite state machines, we build a directed graph whose vertices are the FSMs, and whose arcs are the combinational, also known as Mealy, signals connecting them. We show that it is possible to schedule the order of evaluation of each FSM at compile-time as long as there is no cycle in this graph. We also show that using a topological sort on the graph provides a correct schedule. A system modeled in C including a MIPS R3000 microprocessor core, memories and a few other components interconnected on a PI-Bus simulated using this technique runs at around 150 K cycles per second on a Pentium 120.