Skip to main content
Redhat Developers  Logo
  • Products

    Featured

    • Red Hat Enterprise Linux
      Red Hat Enterprise Linux Icon
    • Red Hat OpenShift AI
      Red Hat OpenShift AI
    • Red Hat Enterprise Linux AI
      Linux icon inside of a brain
    • Image mode for Red Hat Enterprise Linux
      RHEL image mode
    • Red Hat OpenShift
      Openshift icon
    • Red Hat Ansible Automation Platform
      Ansible icon
    • Red Hat Developer Hub
      Developer Hub
    • View All Red Hat Products
    • Linux

      • Red Hat Enterprise Linux
      • Image mode for Red Hat Enterprise Linux
      • Red Hat Universal Base Images (UBI)
    • Java runtimes & frameworks

      • JBoss Enterprise Application Platform
      • Red Hat build of OpenJDK
    • Kubernetes

      • Red Hat OpenShift
      • Microsoft Azure Red Hat OpenShift
      • Red Hat OpenShift Virtualization
      • Red Hat OpenShift Lightspeed
    • Integration & App Connectivity

      • Red Hat Build of Apache Camel
      • Red Hat Service Interconnect
      • Red Hat Connectivity Link
    • AI/ML

      • Red Hat OpenShift AI
      • Red Hat Enterprise Linux AI
    • Automation

      • Red Hat Ansible Automation Platform
      • Red Hat Ansible Lightspeed
    • Developer tools

      • Red Hat Trusted Software Supply Chain
      • Podman Desktop
      • Red Hat OpenShift Dev Spaces
    • Developer Sandbox

      Developer Sandbox
      Try Red Hat products and technologies without setup or configuration fees for 30 days with this shared Openshift and Kubernetes cluster.
    • Try at no cost
  • Technologies

    Featured

    • AI/ML
      AI/ML Icon
    • Linux
      Linux Icon
    • Kubernetes
      Cloud icon
    • Automation
      Automation Icon showing arrows moving in a circle around a gear
    • View All Technologies
    • Programming Languages & Frameworks

      • Java
      • Python
      • JavaScript
    • System Design & Architecture

      • Red Hat architecture and design patterns
      • Microservices
      • Event-Driven Architecture
      • Databases
    • Developer Productivity

      • Developer productivity
      • Developer Tools
      • GitOps
    • Secure Development & Architectures

      • Security
      • Secure coding
    • Platform Engineering

      • DevOps
      • DevSecOps
      • Ansible automation for applications and services
    • Automated Data Processing

      • AI/ML
      • Data Science
      • Apache Kafka on Kubernetes
      • View All Technologies
    • Start exploring in the Developer Sandbox for free

      sandbox graphic
      Try Red Hat's products and technologies without setup or configuration.
    • Try at no cost
  • Learn

    Featured

    • Kubernetes & Cloud Native
      Openshift icon
    • Linux
      Rhel icon
    • Automation
      Ansible cloud icon
    • Java
      Java icon
    • AI/ML
      AI/ML Icon
    • View All Learning Resources

    E-Books

    • GitOps Cookbook
    • Podman in Action
    • Kubernetes Operators
    • The Path to GitOps
    • View All E-books

    Cheat Sheets

    • Linux Commands
    • Bash Commands
    • Git
    • systemd Commands
    • View All Cheat Sheets

    Documentation

    • API Catalog
    • Product Documentation
    • Legacy Documentation
    • Red Hat Learning

      Learning image
      Boost your technical skills to expert-level with the help of interactive lessons offered by various Red Hat Learning programs.
    • Explore Red Hat Learning
  • Developer Sandbox

    Developer Sandbox

    • Access Red Hat’s products and technologies without setup or configuration, and start developing quicker than ever before with our new, no-cost sandbox environments.
    • Explore Developer Sandbox

    Featured Developer Sandbox activities

    • Get started with your Developer Sandbox
    • OpenShift virtualization and application modernization using the Developer Sandbox
    • Explore all Developer Sandbox activities

    Ready to start developing apps?

    • Try at no cost
  • Blog
  • Events
  • Videos

Red Hat at the ISO C++ Standards Meeting (June 2014): Parallelism and Concurrency

August 20, 2014
Torvald Riegel
Related topics:
Developer Tools
Related products:
Developer Tools

Share:

    Recently Red Hat sent several representatives to the JTC1/SC22/WG21 C++ Standards Committee meetings, which were held in June 2014 at the University of Applied Sciences in Rapperswil, Switzerland.

    As in past ISO C++ meetings, SG1, the study group on parallelism and concurrency, met for the whole week to discuss proposals and work on the technical specifications (TS) for both parallelism and concurrency.

    The Parallelism TS seems ready for a first publication soon. SG1 renamed the execution policy that allows vector hardware to be used by the parallel algorithms library to distinguish it from traditional vector execution as found on many current CPUs; this change clarifies that this policy allows both vector and task parallelism to be used, and allows for a vector-only policy to be added later on. SG1 also discussed a proposal about abstractions for task parallelism, as well as a few other proposals related to parallelism.

    On the concurrency side, SG1 discussed a few proposals related to synchronization, for example one about latches and barriers, which SG1 wants to be added to the Concurrency TS after further review. The other large group of concurrency-related proposals all revolve around the concurrent execution of tasks, and how to let programmers control that. The proposals about coroutines and "Resumable Functions" have in common that they both are based on, conceptually, userspace threads; I've argued that this common part should be unified in these proposals, and I think we made initial progress toward this during the meeting. Finally, we discussed a couple of other proposals that suggested changes to or alternatives for the Executor facilities in the Concurrency TS; in the end, there wasn't enough agreement about the current Executor feature anymore and SG1 agreed to remove this part from the Concurrency TS for now.

    I presented two papers at the meeting. First, a proposal about light-weight execution agents (EAs), which are an abstract notion used to specify how code is executed in parallel or concurrently. Specifically, I define three kinds of EAs that have different forward progress guarantees (i.e., guarantees for when and under which circumstances an EA will execute a piece of code): concurrent, parallel, and weakly parallel. Concurrent EAs always eventually make progress, no matter what other EAs are or are not doing; thus, this is the same guarantee that OS threads provide under typical general-purpose OS schedulers. Parallel EAs give weaker guarantees, which resemble what a thread pool with a bounded number of threads would provide: once such an EA starts executing (e.g., an iteration of a parallel loop), it will behave like a thread; but there is no guarantee that such an EA will get started concurrently with another EA (because there might not be any thread left in the pool to start the EA). Finally, weakly parallel EAs provide even weaker progress guarantees but allow code to be executed with vector instructions. The benefit of these definitions is that (1) they give precise rules to the programmer about what kinds of synchronization are allowed in parallel tasks (e.g., can one safely use a mutex that blocks until another EA makes progress?) and (2) make it clear what implementations have to provide (e.g., can a parallel loop be implemented using a bounded thread pool?). These definitions are not yet part of any of the technical specifications, but there was general agreement in SG1 at the meeting that forward progress guarantees are an essential part of specifying parallel or concurrent execution.

    Second, I presented a paper about memory_order_consume that I co-authored together with Paul McKenney (of Read-Copy-Update fame) and Jeff Preshing. memory_order_consume is a feature in C++11 and C11 that is supposed to allow synchronization experts to rely on ordering due to data dependencies instead of having to use explicit hardware barriers. This is heavily used in the Linux kernel's Read-Copy-Update implementation, for example, and can decrease the cost of synchronization on Power and ARM CPUs. The problem with memory_order_consume is that the way it is specified in the current standard makes it impractical to implement in a compiler, and we are not aware of any optimized implementations of it. In the paper, we analyze why this is the case and propose alternative semantics that seem practical to implement. There is consensus in SG1 that we need to fix memory_order_consume, and while our paper is a first step towards a better memory_order_consume, we got good feedback and even a suggestion for one further potential solution in the meeting.

    We're always interested to hear what matters to Red Hat Enterprise Linux developers, so if you have comments or questions on any aspects of the upcoming C++ standards - in the concurrency area, or otherwise - please feel free to get in touch with us at rheldevelop AT redhat DOT com or @RHELdevelop.

    Editor's note:  a related article on C++ Core and Library is also available.

    Last updated: April 5, 2018

    Recent Posts

    • How to run a fraud detection AI model on RHEL CVMs

    • How we use software provenance at Red Hat

    • Alternatives to creating bootc images from scratch

    • How to update OpenStack Services on OpenShift

    • How to integrate vLLM inference into your macOS and iOS apps

    Red Hat Developers logo LinkedIn YouTube Twitter Facebook

    Products

    • Red Hat Enterprise Linux
    • Red Hat OpenShift
    • Red Hat Ansible Automation Platform

    Build

    • Developer Sandbox
    • Developer Tools
    • Interactive Tutorials
    • API Catalog

    Quicklinks

    • Learning Resources
    • E-books
    • Cheat Sheets
    • Blog
    • Events
    • Newsletter

    Communicate

    • About us
    • Contact sales
    • Find a partner
    • Report a website issue
    • Site Status Dashboard
    • Report a security problem

    RED HAT DEVELOPER

    Build here. Go anywhere.

    We serve the builders. The problem solvers who create careers with code.

    Join us if you’re a developer, software engineer, web designer, front-end designer, UX designer, computer scientist, architect, tester, product manager, project manager or team lead.

    Sign me up

    Red Hat legal and privacy links

    • About Red Hat
    • Jobs
    • Events
    • Locations
    • Contact Red Hat
    • Red Hat Blog
    • Inclusion at Red Hat
    • Cool Stuff Store
    • Red Hat Summit

    Red Hat legal and privacy links

    • Privacy statement
    • Terms of use
    • All policies and guidelines
    • Digital accessibility

    Report a website issue