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ALF Research team

Amdahl's Law is Forever

  • Leader : Andre Seznec
  • Research center(s) : CRI Rennes - Bretagne Atlantique
  • Field : Algorithmics, Programming, Software and Architecture
  • Theme : Architecture, Languages and Compilation
  • Partner(s) : Université Rennes 1
  • Collaborator(s) : Institut de recherche en informatique et systèmes aléatoires (IRISA) (UMR6074)

Team presentation

Multicore processors have now become mainstream for both general-purpose and embedded computing. In the near future, every hardware platform will feature thread level parallelism. Therefore, the overall computer science research community, but also industry, is facing new challenges; the parallel architectures will have to be exploited by every application from HPC computing, web and entreprise servers, but also PCs, smartphones and ubiquitous embedded systems.

Within a decade, it will become technologically feasible to implement 1000's of cores on a single chip. However, several challenges must be addressed to allow the end-user to benefit from these 1000's cores chip. At that time, most applications will not be fully parallelized, therefore the effective performance of most computer systems will strongly depend on their performance on sequential sections and sequential control threads: Amdahl's law is forever. Parallel applications will not become mainstream if they have to be adapted to each new platform, therefore a simple performance scalability / portability path is needed for these applications. In many application domains, in particular real-time systems, the effective use of multicore chips will depend on the ability of the software and hardware providers to accurately assess the performance of applications.

The ALF team will regroup researchers in computer architecture, software/compiler optimization, and real-time systems. The long-term goal of the ALF project-team is to allow the end-user to benefit from the 2020's many-core platform. We will address this issue through architecture, i.e. we intend to influence the definition of the 2020's many-core architecture, compiler, i.e. we intend to provide new code generation techniques for efficient execution on many-core architectures and performance prediction/guarantee, i.e. we intend to propose to predict/guarantee the response time of many-core architectures.

Research themes

  • Microarchitecture research directions: High performance on single process and sequential codes is one of the key issues for enabling overall high performance on a 1000's core system. Therefore we anticipate that future manycore architectures will feature heterogeneous design with many simple cores and a few complex cores. Therefore the research in the ALF project focuses on refining the microarchitecture to achieve high performance on single process and/or sequential code sections. We focus our architecture research in two main directions 1) enhancing the microarchitecture of high-end superscalar processors, 2) exploiting/modifying heterogeneous multicore architecture on a single process. We also tackle a technological/architecture issue, the temperature wall.
  • Compilers are keystone solutions for any approach that deals with high performance on 100+ processors systems. But general-purpose compilers try to embrace so many domains and try to serve so many constraints that they frequently fail to achieve very high performance. They need to be deeply revisited. We identify four main compiler/software related issues that must be addressed in order to allow efficient use of multi- and many-cores: 1) programming 2) resource management 3) application deployment 4) portable performance. Addressing these challenges require to revisit parallel programming and code generation extensively.
  • Performance predictability for real-time systems} While compiler and architecture research efforts often focus on maximizing average case performance, applications with real-time constraints do not only need high performance but also performance guarantees in all situations, including the worst-case situation. Worst-Case Execution Time estimates (WCET) need to be upper bounds of any possible execution times. The amount of safety required depends on the criticality of applications. Within the ALF team, our objective is to study performance guarantees for both (i) sequential codes running on complex cores ; (ii) parallel codes running on the multicores.

International and industrial relations

  • Member of the HIPEAC3 network
  • ERC Advanced Grant DAL, André Seznec
  • Member of ANR WSEPT project
  • Intel research grant
  • Member of the Nano 2017 project  PSAIC
  • Member of the COST Action TACle
  • Member of the PIA project Capacités

Keywords: Computer architecture Multicore Compilation