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

OpenJDK and Containers

April 4, 2017
Christine Flood
Related topics:
ContainersDeveloper ToolsJava
Related products:
Red Hat build of OpenJDK

Share:

    What can be done to help the OpenJDK JVM play well in the world of Linux Containers?
    I thought I'd start tackling this issue by answering some frequently asked questions:

    Why is it when I specify -Xmx=1g my JVM uses up more memory than 1gb of memory?

    Specifying -Xmx=1g is telling the JVM to allocate a 1gb heap. It's not telling the JVM to limit its entire memory usage to 1gb. There are card tables, code caches, and all sorts of other off heap data structures. The parameter you use to specify total memory usage is -XX:MaxRAM. Be aware that with -XX:MaxRam=500m your heap will be approximately 250mb.

    Why is it when I specify -m 10m to my Linux container the JVM appears to ignore the limit?

    The JVM historically looked in /proc to figure out how much memory was available and then set its heap size based on that value. Unfortunately, containers like Docker don't provide container specific information in /proc. I've proposed a patch which has been accepted upstream to provide a -XX:+UseCGroupMemoryLimitForHeap command line argument which tells the JVM to look in /sys/fs/cgroup/memory/memory.limit_in_bytes to figure out how much memory is available. If this patch isn't available in the OpenJDK version you are running you can simulate it by setting -XX:MaxRAM=n explicitly.

    What if I specify cpusets?

    There is a patch in OpenJDK8, which will use the information available to the cgroup to calculate the appropriate number of parallel GC threads. However, if this patch is not available in your version of OpenJDK you may end up with 8 parallel GC threads and only 2 cpus in your container. The workaround is to specify the number of parallel gc threads explicitly. -XX:ParallelGCThreads=2.  If you only have 1 CPU in your container I highly recommend that you run with -XX:+UseSerialGC and avoid parallel GC altogether.

    OK, so I know I can explicitly set things like heap size and parallel gc threads, but how can I tell the JVM I don't care about pause time or throughput I just want it to use as few resources as possible?

    -XX:+UseSerialGC will run with only 1 garbage collection thread and will run with the smallest heap overhead.
    -XX:+TieredCompilation -XX:TieredStopAtLevel=1 will disable the optimizing compiler and save some space.

    My program has a startup phase where it needs a lot of heap but will settle into a quiet looping phase where it doesn't need as much. Can I configure the heap to grow, shrink, and give memory it isn't currently using back to the operating system?

    SerialGC will do this for you, but you can ask it to be more aggressive.
    -XX:MinHeapFreeRatio=20 (This defaults to Grow when the heap is greater than 80% occupied)
    -XX:MaxHeapFreeRatio=40 (Shrink when the heap is less than 60% occupied).
    Parallel GC will do this for you as well. We recommend the following additional parameters:
    -XX:GCTimeRatio=4
    -XX:AdaptiveSizePolicyWeight=90

    OK, I have a 2gb heap, and my jvm instance is using 4gb, where are the other 2gb going?

    You can track native memory by running Java with the following command line arguments:

    java -XX:+UnlockDiagnosticVMOptions -XX:NativeMemoryTracking="summary" -XX:+PrintNMTStatistics

    You can decrease some of these parameters.  If you are running with many threads, for example, decreasing the size of your Java thread stacks might help.  -Xss228k will decrease the size of your Java stacks.

    JVM Argument Effect
    -XX:+UseSerialGC Uses only 1 GC thread. This both limits the cpu instruction count for garbage collection, but also results in the minimal memory footprint.
    -XX:MaxRAM=n Sets the maximum amount of memory used by the JVM to n, where n may be expressed in terms of megabytes 100m or gigabytes 2g.
    -XX:+UseCGroupMemoryLimitForHeap This flag present in the more recent builds of JDK8 tells the JVM to use the information in /sys/fs/cgroup/memory/memory.limit_in_bytes to calculate memory defaults.
    -XX:ParallelGCThreads=n Set the number of parallel GC threads to n. This is helpful if you are trying to limit the cpu usage of your container, or if you are running with a JVM that doesn't include the patch to calculate GC threads based on processors available to the cgroup.
    -XX:+TieredCompilation
    -XX:TieredStopAtLevel=1
    Turns off the optimizing compiler. This can sometimes decrease the footprint of your running JVM.
    -XX:MinHeapFreeRatio=20
    -XX:MaxHeapFreeRatio=40
    These parameters tell the heap to shrink aggressively and to grow conservatively.  Thereby optimizing the amount of memory available to the operating system.
    -XX:GCTimeRatio=4
    -XX:AdaptiveSizePolicyWeight=90
    These parameters are necessary when running parallel GC if you want to use the Min and Max Heap Free ratios.
    -XX:+UnlockDiagnosticVMOptions
    -XX:NativeMemoryTracking=
    "summary"
    -XX:+PrintNMTStatistics
    These options will print out the non-heap memory usage of your JVM.
    -Xss228k This will decrease the size of your Java Stacks.

    If you have any additional questions, please drop me an email: chf@redhat.com.


    Download the OpenJDK and accept the terms and conditions of the Red Hat Developer Program, which provides no-cost subscriptions for development use only.

    Last updated: April 22, 2022

    Recent Posts

    • More Essential AI tutorials for Node.js Developers

    • 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

    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