XXConcGCThreads

XXConcGCThreads refers to the -XX:ConcGCThreads VM option in the Oracle HotSpot Java Virtual Machine. It controls the number of concurrent garbage collector (GC) threads that participate in the GC’s concurrent phases. The flag is used by collectors that perform background work, notably the Concurrent Mark Sweep (CMS) collector and the Garbage-First (G1) collector, and is set with -XX:ConcGCThreads=<n>.

Setting a higher value can allow more GC work to progress in parallel with the running application, which may reduce pause times on multi-core systems and improve throughput in some workloads. However, increasing the number of concurrent GC threads can also raise CPU contention between the application and the GC, potentially reducing overall performance if the value is too high. The optimal setting depends on the workload characteristics and the available hardware.

Defaults and compatibility vary by GC algorithm and VM version. CMS has been deprecated in newer Java releases and may be ignored if CMS is not enabled. For G1, -XX:ConcGCThreads influences the number of concurrent worker threads during phases such as marking and cleanup, but modern JVM tuning often relies on collector-specific options in addition to or instead of this flag. Users should consult their JDK version’s documentation to understand current behavior and defaults for their environment.

Example: java -XX:ConcGCThreads=4 -jar myapp.jar

See also: -XX:ParallelGCThreads, -XX:+UseG1GC, -XX:+UseConcMarkSweepGC.