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The Z Garbage Collector, also known as ZGC, (ZGC) is a scalable low latency garbage collector designed to meet the following goals:

  • Sub-millisecond max pause times
  • Pause times do not increase with the heap, live-set or root-set size
  • Handle heaps ranging from a 8MB to 16TB in size

    . ZGC performs all expensive work concurrently, without stopping the execution of application threads for more than a millisecond. It is suitable for applications which require low latency. Pause times are independent of the heap size that is being used. ZGC works well with heap sizes from a few hundred megabytes to 16TB. 

    ZGC was initially introduced as an experimental feature in JDK 11, and was declared Production Ready in JDK 15. In JDK 21 was reimplemented to support generations.

    At a glance, ZGC is:

    • Concurrent
    • Region-based
    • Compacting
    • NUMA-aware
    • Using colored pointers
    • Using load barriers
    • Using store barriers (in the generational mode)

    At its core, ZGC is a concurrent garbage collector, meaning all heavy lifting work is done while Java threads continue to execute. This greatly limits the impact garbage collection will have on your application's response time.

    This OpenJDK project is sponsored by the HotSpot Group.

    Contents

    Table of Contents

    Supported Platforms

    PlatformSupportedSinceComment
    Linux/x64(tick)JDK 15 (Experimental since JDK 11)
    Linux/AArch64(tick)JDK 15 (Experimental since JDK 13)
    Linux/PowerPC(tick)JDK 18
    macOS/x64(tick)JDK 15 (Experimental since JDK 14)
    macOS/AArch64(tick)JDK 17
    Windows/x64(tick)JDK 15 (Experimental since JDK 14)Requires Windows version 1803 (Windows 10 or Windows Server 2019) or later.
    Windows/AArch64(tick)JDK 16

    Quick Start

    If you're trying out ZGC for the first time, start by using the following GC options:

    Code Block
    -XX:+UseZGC -Xmx<size> -Xlog:gc

    For more detailed logging, use the following options:

    Code Block
    -XX:+UseZGC -Xmx<size> -Xlog:gc*

    See below for more information on these and additional options.

    Configuration & Tuning

    Overview

    The following JVM options can be used with ZGC:

    General GC OptionsZGC OptionsZGC Diagnostic Options (-XX:+UnlockDiagnosticVMOptions)
    -XX:MinHeapSize, -Xms
    -XX:InitialHeapSize, -Xms
    -XX:MaxHeapSize, -Xmx
    -XX:SoftMaxHeapSize
    -XX:ConcGCThreads
    -XX:ParallelGCThreads
    -XX:UseDynamicNumberOfGCThreads
    -XX:UseLargePages
    -XX:UseTransparentHugePages
    -XX:UseNUMA
    -XX:SoftRefLRUPolicyMSPerMB
    -XX:AllocateHeapAt

    -XX:ZAllocationSpikeTolerance

    -XX:ZCollectionInterval

    -XX:ZFragmentationLimit

    -XX:ZMarkStackSpaceLimit

    -XX:ZProactive

    -XX:ZUncommit

    -XX:ZUncommitDelay

    -XX:ZStatisticsInterval

    -XX:ZVerifyForwarding

    -XX:ZVerifyMarking

    -XX:ZVerifyObjects

    -XX:ZVerifyRoots

    -XX:ZVerifyViews

    Note for JDK 21 you need to explicitly use the following options to use the newer, generational mode of ZGC:

    Code Block
    -XX:+UseZGC -XX:+ZGenerational 


    See below for more information on these and additional options.

    Configuration & Tuning

    ZGC has been designed to be adaptive and to require minimal manual configuration. During the execution of the Java program, ZGC dynamically adapts to the workload by resizing generations, scaling the number of GC threads, and adjusting tenuring thresholds. The main tuning knob is to increase the maximum heap size.

    Info
    titleJDK 21

    In JDK 21, ZGC comes in two versions: The new, generational version and the legacy, non-generational version. The Non-generational ZGC is the older version of ZGC, which doesn't take advantage of generations (see Generations) to optimize its runtime characteristics. It is encouraged that users transition to use the newer Generational ZGC.

    The Generational ZGC is enabled with the command-line options -XX:+UseZGC -XX:+ZGenerational.

    The Non-generational ZGC is enabled with the command-line option -XX:+UseZGC.

    Overview

    The following JVM options can be used with ZGC:

    General GC OptionsZGC OptionsZGC Diagnostic Options (-XX:+UnlockDiagnosticVMOptions)
    -XX:MinHeapSize, -Xms
    -XX:InitialHeapSize, -Xms
    -XX:MaxHeapSize, -Xmx
    -XX:SoftMaxHeapSize
    -XX:ConcGCThreads
    -XX:ParallelGCThreads
    -XX:UseDynamicNumberOfGCThreads
    -XX:UseLargePages
    -XX:UseTransparentHugePages
    -XX:UseNUMA
    -XX:SoftRefLRUPolicyMSPerMB
    -XX:AllocateHeapAt

    -XX:ZAllocationSpikeTolerance

    -XX:ZCollectionInterval

    -XX:ZFragmentationLimit

    -XX:ZMarkStackSpaceLimit

    -XX:ZProactive

    -XX:ZUncommit

    -XX:ZUncommitDelay

    -XX:ZStatisticsInterval

    -XX:ZVerifyForwarding

    -XX:ZVerifyMarking

    -XX:ZVerifyObjects

    -XX:ZVerifyRoots

    -XX:ZVerifyViews

    -XX:ZYoungGCThreads

    -XX:ZOldGCThreads

    -XX:ZBufferStoreBarriers


    In addition to these the following flags are available when the generational mode is enabled with -XX:+UseZGC -XX:+ZGenerational:

    General GC OptionsZGC OptionsZGC Diagnostic Options (-XX:+UnlockDiagnosticVMOptions)

    -XX:ZCollectionIntervalMinor

    -XX:ZCollectionIntervalMajor

    -XX:ZYoungCompactionLimit


    -XX:ZVerifyRemembered

    -XX:ZYoungGCThreads

    -XX:ZOldGCThreads

    -XX:ZBufferStoreBarriers

    Setting Heap Size

    The most important tuning option for ZGC is setting the maximum heap size, which you can set with the -Xmx command-line option. Because ZGC is a concurrent collector, you must select a maximum heap size such that the heap can accommodate the live-set of your application and there is enough headroom in the heap to allow allocations to be serviced while the GC is running. How much headroom is needed very much depends on the allocation rate and the live-set size of the application. In general, the more memory you give to ZGC the better. But at the same time, wasting memory is undesirable, so it’s all about finding a balance between memory usage and how often the GC needs to run.

    ZGC has another command-line option related to the heap size named -XX:SoftMaxHeapSize. It can be used to set a soft limit on how large the Java heap can grow. ZGC will strive to not grow beyond this limit, but is still allowed to grow beyond this limit up to the maximum heap size. ZGC will only use more than the soft limit if that is needed to prevent the Java application from stalling and waiting for the GC to reclaim memory. For example, with the command-line options -Xmx5g -XX:SoftMaxHeapSize=4g ZGC will use 4GB as the limit for its heuristics, but if it can't keep the heap size below 4GB it is still allowed to temporarily use up to 5GB.

    Setting Concurrent GC Threads

    Note! This section pertain to the non-generational version of ZGC. Generational ZGC has a more adaptive implementation and you are less likely to need to tweak the GC threads.

    The second tuning option one might want to look at is setting the number of concurrent GC threads (-XX:ConcGCThreads=<number>). ZGC has heuristics to automatically select this number. This heuristic usually works well but depending on the characteristics of the application this might need to be adjusted. This option essentially dictates how much CPU-time the GC should be given. Give it too much and the GC will steal too much CPU-time from the application. Give it too little, and the application might allocate garbage faster than the GC can collect it.

    NOTE!! Starting from JDK 17, ZGC dynamically scales up and down the number of concurrent GC threads. This makes it even more unlikely that you'd need to adjust the concurrent number of GC threads.

    NOTE!!! In general, if low latency (i.e. low application response time) is important for you application, then never over-provision your system. Ideally, your system should never have more than 70% CPU utilization.

    Returning Unused Memory to the Operating System

    By default, ZGC uncommits unused memory, returning it to the operating system. This is useful for applications and environments where memory footprint is a concern, but might have a negative impact on the latency of Java threads. You can disable this feature with the command-line option -XX:-ZUncommit. Furthermore, memory will not be uncommitted so that the heap size shrinks below the minimum heap size (-Xms). This means this feature will be implicitly disabled if the minimum heap size (-Xms) is configured to be equal to the maximum heap size (-Xmx).

    You can configure an uncommit delay using -XX:ZUncommitDelay=<seconds> (default is 300 seconds). This delay specifies for how long memory should have been unused before it's eligible for uncommit.

    NOTE! Allowing the GC to commit and uncommit memory while the application is running could have a negative impact on the latency of Java threads. If extremely low latency is the main reason for running with ZGC, consider running with the same value for -Xmx and -Xms, and use -XX:+AlwaysPreTouch to page in memory before the application starts.

    NOTE!

    Enabling ZGC

    Use the -XX:+UseZGC options to enable ZGC.

    Setting Heap Size

    The most important tuning option for ZGC is setting the max heap size (-Xmx<size>). Since ZGC is a concurrent collector a max heap size must be selected such that, 1) the heap can accommodate the live-set of your application, and 2) there is enough headroom in the heap to allow allocations to be serviced while the GC is running. How much headroom is needed very much depends on the allocation rate and the live-set size of the application. In general, the more memory you give to ZGC the better. But at the same time, wasting memory is undesirable, so it’s all about finding a balance between memory usage and how often the GC needs to run.

    Setting Concurrent GC Threads

    The second tuning option one might want to look at is setting the number of concurrent GC threads (-XX:ConcGCThreads=<number>). ZGC has heuristics to automatically select this number. This heuristic usually works well but depending on the characteristics of the application this might need to be adjusted. This option essentially dictates how much CPU-time the GC should be given. Give it too much and the GC will steal too much CPU-time from the application. Give it too little, and the application might allocate garbage faster than the GC can collect it.

    NOTE! In general, if low latency (i.e. low application response time) is important for you application, then never over-provision your system. Ideally, your system should never have more than 70% CPU utilization.

    Returning Unused Memory to the Operating System

    By default, ZGC uncommits unused memory, returning it to the operating system. This is useful for applications and environments where memory footprint is a concern. This feature can be disabled using -XX:-ZUncommit. Furthermore, memory will not be uncommitted so that the heap size shrinks below the minimum heap size (-Xms). This means this feature will be implicitly disabled if the minimum heap size (-Xms) is configured to be equal to the maximum heap size (-Xmx).

    An uncommit delay can be configured using -XX:ZUncommitDelay=<seconds> (default is 300 seconds). This delay specifies for how long memory should have been unused before it's eligible for uncommit.

    NOTE! On Linux, uncommitting unused memory requires fallocate(2) with FALLOC_FL_PUNCH_HOLE support, which first appeared in kernel version 3.5 (for tmpfs) and 4.3 (for hugetlbfs).

    Enabling

    Using Large Pages

    On Linux

    Configuring ZGC to use large pages will generally yield better performance (in terms of throughput, latency and start up time) and comes with no real disadvantage, except that it's slightly more complicated to setup. The setup process typically requires root privileges, which is why it's not enabled by default.

    Enabling Large Pages On Linux

    On Linux / x86, large pages (also known as "huge pages") have a size of 2MB.

    Let's assume you want a 16G 16GB Java heap. That means you need 16G 16GB / 2M 2MB = 8192 huge pages.

    First assign The heap requires at least 16G 16GB (8192 pages) of memory to the pool of huge pages. The "at least" part is important, since enabling the use of large pages in the JVM means that not only the GC will try to use these for the Java heap, but also that the pool of huge pages. The heap along with other parts of the JVM will try to use them large pages for various internal data structures (such as code heap , and marking bitmaps, etc). In this example we you will therefore reserve 9216 pages (18G18GB) to allow for 2G 2GB of non-Java heap allocations to use large pages.

    Configure the system's huge page pool to have the required number of pages (requires root privileges):

    Code Block
    $ echo 9216 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

    Note that the above command is not guaranteed to be successful if the kernel can not cannot find enough free huge pages to satisfy the request. Also note that it might take some time for the kernel to process the request. Before proceeding, check the number of huge pages assigned to the pool to make sure the request was successful and has completed.

    Code Block
    $ cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
    9216 9216

    NOTE! If you're using a Linux kernel >= 4.14, then the next step (where you mount a hugetlbfs filesystem) can be skipped. However, if you're using an older kernel then ZGC needs to access large pages through a hugetlbfs filesystem.

    Mount a hugetlbfs filesystem (requires root privileges) and make it accessible to the user running the JVM (in this example we're assuming this user has 123 as its uid).

    Code Block
    $ mkdir /hugepages
    $ mount -t hugetlbfs -o uid=123 nodev /hugepages 

    Now start the JVM using the -XX:+UseLargePages option.

    Code Block
    $ java -XX:+UseZGC -Xms16G -Xmx16G -XX:+UseLargePages ...

    If there are more than one accessible hugetlbfs filesystem available, then (and only then) do you also have to use -XX:AllocateHeapAt to specify the path to the filesystems you want to use. For example, assume there are multiple accessible hugetlbfs filesystems mounted, but the filesystem you specifically want to use it mounted on /hugepages, then use the following options.

    Code Block
    $ java -XX:+UseZGC -Xms16G -Xmx16G -XX:+UseLargePages -XX:AllocateHeapAt=/hugepages ...

    NOTE! The configuration of the huge page pool and the mounting of the hugetlbfs file system is not persistent across reboots, unless adequate measures are taken.

    Enabling Transparent Huge Pages On Linux

    An alternative to using explicit large pages (as described above) is to use

    NOTE! On Linux, using ZGC with transparent huge pages enabled requires kernel >= 4.

    Use of transparent huge pages is usually not recommended for latency sensitive applications, because it tends to cause unwanted latency spikes. However, it might be worth experimenting with to see if/how your workload is affected by it. But be aware, your mileage may vary.

    NOTE! On Linux, using ZGC with transparent huge pages enabled requires kernel >= 4.7.

    Use the following options to enable transparent huge pages in the VM:

    Code Block
    -XX:+UseLargePages -XX:+UseTransparentHugePages

    These options tell the JVM to issue madvise(..., MADV_HUGEPAGE) calls for memory it mapps, which is useful when using transparent huge pages in madvise mode.

    To enable transparent huge pages you also need to configure the kernel, by enabling the madvise mode.

    Code Block$ echo madvise >

    7.'

    Use the following options to enable transparent huge pages in the VM:

    Code Block
    -XX:+UseLargePages -XX:+UseTransparentHugePages

    These options tell the JVM to issue madvise(..., MADV_HUGEPAGE) calls for memory it maps, which is useful when using transparent huge pages in madvise mode.

    To enable transparent huge pages, you also need to configure the kernel by enabling madvise mode.

    Code Block
    $ echo madvise > /sys/kernel/mm/transparent_hugepage/enabled

    ZGC uses shmem huge pages for the heap, so the following kernel setting also needs to be configured:

    Code Block
    $ echo advise > /sys/kernel/mm/transparent_hugepage/shmem_enabled

    It is important to check these kernel settings when comparing the performance of different GCs. Some Linux distributions forcefully enable transparent huge pages for private pages by configuring /sys/kernel/mm/transparent_hugepage/enabled

    and

    Code Block$ echo advise >

    to be set to always, while leaving /sys/kernel/mm/transparent_hugepage/shmem_enabled

    See the kernel documentation at the default never. In this case all GCs but ZGC will make use of transparent huge pages for the heap. See Transparent Hugepage Support for more information.

    Enabling NUMA Support

    ZGC has NUMA support, which means it will try it's best to direct Java heap allocations to NUMA-local memory. This feature is enabled by default. However, it will automatically be disabled if the JVM detects that it's bound to only use memory on a single NUMA node. In general, you don't need to worry about this setting, but if you want to explicitly override the JVM's decision you can do so by using the -XX:+UseNUMA or -XX:-UseNUMA options.

    When running on a NUMA machine (e.g. a multi-socket x86 machine), having NUMA support enabled will often give a noticeable performance boost.

    Enabling GC Logging

    GC logging is enabled using the following command-line option:

    Code Block
    -Xlog:<tag set>,[<tag set>, ...]:<log file>

    For general information/help on this option:

    Code Block
    -Xlog:help

    To enable basic logging (one line of output per GC):

    Code Block
    -Xlog:gc:gc.log

    To enable GC logging that is useful for tuning/performance analysis:

    Code Block
    -Xlog:gc*:gc.log

    Where gc* means log all tag combinations that contain the gc tag, and :gc.log means write the log to a file named gc.log.

    gc.log

    Where gc* means log all tag combinations that contain the gc tag, and :gc.log means write the log to a file named gc.log.

    Change Log

    JDK 24

    • Removed the non-generational mode of ZGC (JEP 490)

    JDK 23

    • Make Generational ZGC the default ZGC version (and deprecate non-generational ZGC) (JEP 474)
    • Latency Issue Due to ICBufferFull Safepoints Resolved (JDK-8322630) 

    JDK 21

    • Support for generations (-XX:+ZGenerational) (JEP 439)
    Change Log

    JDK 18

    • Support for String Deduplication (-XX:+UseStringDeduplication)
    • Linux/PowerPC support
    • Various bugfixes bug-fixes and optimizations

    JDK 17

    • Dynamic Number of GC threads
    • Reduced mark stack memory usage
    • macOS/aarch64 support
    • GarbageCollectorMXBeans for both pauses and cycles
    • Fast JVM termination

    JDK 16

    • Concurrent Thread Stack Scanning (JEP 376)
    • Support for in-place relocation
    • Performance improvements (allocation/initialization of forwarding tables, etc)

    JDK 15

    • Production ready (JEP 377)
    • Improved NUMA awareness
    • Improved allocation concurrency
    • Support for Class Data Sharing (CDS)
    • Support for placing the heap on NVRAM
    • Support for compressed class pointers
    • Support for incremental uncommit
    • Fixed support for transparent huge pages
    • Additional JFR events

    JDK 14

    • macOS support (JEP 364)
    • Windows support (JEP 365)
    • Support for tiny/small heaps (down to 8M)
    • Support for JFR leak profiler
    • Support for limited and discontiguous address space
    • Parallel pre-touch (when using -XX:+AlwaysPreTouch)
    • Performance improvements (clone intrinsic, etc)
    • Stability improvements

    JDK 13

    • Increased max heap size from 4TB to 16TB
    • Support for uncommitting unused memory (JEP 351)
    • Support for -XX:SoftMaxHeapSIze
    • Support for the Linux/AArch64 platform
    • Reduced Time-To-Safepoint

    JDK 12

    • Support for concurrent class unloading
    • Further pause time reductions

    JDK 11

    • Initial version of ZGC
    • Does not support class unloading (using -XX:+ClassUnloading has no effect)

    FAQ

    What does the "Z" in ZGC stand for?

    It doesn't stand for anything, ZGC is just a name. It was originally inspired by, or a homage to, ZFS (the filesystem) which in many ways was revolutionary when it first came out. Originally, ZFS was an acronym for "Zettabyte File System", but that meaning was abandoned and it was later said to not stand for anything. It's just a name. See Jeff Bonwick's Blog for more details.

    Is it pronounced "zed gee see" or "zee gee see"?

    There's no preferred pronunciation, both are fine.

    Tip
    titleDownload

    Latest StableReleased: JDK 18
    Latest Development: JDK 19 Early Access24

    Stable: github.com/openjdk/jdk
    Info
    titleSource Code

    Development: github.com/openjdk/zgc/tree/zgc_generational/jdk

    Info
    titleBlog Posts

    How ZGC allocates memory for the Java heap
    ZGC | What's new in JDK 18
    ZGC | What's new in JDK 17
    ZGC | What's new in JDK 16
    ZGC | What's new in JDK 15
    ZGC | Using -XX:SoftMaxHeapSize

    ZGC | What's new in JDK 14

    Compact Forwarding Information
    How do 'hot and cold' objects behave?

    Info
    titleTalks/Presentations/Podcasts
    JVMLS 2024 - Video (48 min)
    JVMLS 2023 - Video (33 min)
    Oracle Developer Live 2022 - Slides | Video (28 min)
    Jokerconf 2021 - Slides
    Inside Java Podcast - ZGC - Sound (30 min)
    Oracle Developer Live 2020 - Slides | Video (40 min)
    Oracle Code One 2019 - Slides
    PL-Meetup 2019 - Slides
    Jfokus 2019 - Slides | Video (21 min)
    Devoxx 2018 - Slides | Video (40 min)
    Oracle Code One 2018 - Slides | Video (45 min)
    Jfokus 2018 - Slides | Video (45 min)
    FOSDEM 2018 - Slides
    Info
    titleMailing List

    Subscribe | Archive

    Info
    titleProject

    Members
    JIRA Dashboard

    JEPs 333, 351, 364, 365, 376, 377, 439, 474, 490


     

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