9 The Z Garbage Collector

The Z Garbage Collector (ZGC) is a scalable low latency garbage collector. 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 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.

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.

Setting the 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.

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.


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.

Using Large Pages

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 16GB Java heap. That means you need 16GB / 2MB = 8192 huge pages.

The heap requires at least 16GB (8192 pages) of memory to the pool of huge pages. The heap along with other parts of the JVM will use large pages for various internal data structures (such as code heap and marking bitmaps). In this example you will reserve 9216 pages (18GB) to allow for 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):

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

Note that the above command is not guaranteed to be successful if the kernel 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.

$ cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages


Enabling Transparent Huge Pages On Linux

An alternative to using explicit large pages (as described previously) is to use transparent huge pages. 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 or how your workload is affected by it.


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:

-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.

$ 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:

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