Pause-less Garbage Collection and Guarded Storage

The Java runtime supports a built-in memory manager that tracks an application’s usage of the Java heap. Garbage Collection (GC) is a process that is triggered when the heap becomes exhausted. During the GC process, the application is stalled in “stop-the-world” phases, to allow GC to identify and reclaim memory from Java objects that are no longer referenced. Untuned, such “stop-the-world” GC phases can have a detrimental impact to your application’s throughput and response time.
Pause-less Garbage Collection (GC) is a new GC mode in the 64-bit IBM SDK for Java, aimed to reduce the impact of GC “stop-the-world” phases, and improve the throughput and consistency of response-times for Java applications. This technology leverages the new Guarded Storage Facility in IBM z14 hardware, to allow more parallel execution of GC-related processing with application code. Pause-less Garbage Collection is particularly relevant for applications with strict response-time Service Level Agreements (SLAs) and/or large Java heaps.

How To Enable
Please see the Requirements section below for the pre-requisite hardware and software levels to support Pause-less Garbage Collection. The Pause-less GC mode is not enabled by default. To enable the new Pause-less GC mode in your application, please introduce -Xgc:concurrentScavenge to the JVM options.

To verify that Pause-less GC is enabled correctly, collect a verbose GC diagnostics with:

      java -verbose:gc –version

and confirm the following attribute is set to true:

      <attribute name=”concurrentScavenger” value=”true” />


  • IBM Java (Java 8 SR5) or newer (64-bit)
  • IBM z14 with the Guarded Storage Facility enabled
  • z/OS 2.3 or z/OS 2.2 with APAR OA51643
  • z/VM 6.4 or newer

The pause-less GC mode is currently only available on 64-bit IBM SDK for z/OS.
Linux kernel support has been upstreamed and awaiting Linux distribution adoption.

Learn more
For more information about Pause-less GC please visit the following:

Join The Discussion

Your email address will not be published. Required fields are marked *