java - Does GC release back memory to OS?

Question

When the garbage collector runs and releases memory does this memory go back to the OS or is it being kept as part of the process. I was under the strong impression that the memory is never actually released back to OS but kept as part of the memory area/pool to be reused by the same process.

As a result the actual memory of a process would never decrease. An article that reminded me was this and Java’s Runtime is written in C/C++ so I guess the same thing applies?

Update
My question is about Java. I am mentioning C/C++ since I assume the Java’s allocation/deallocation is done by JRE using some form of malloc/delete

Answer 1

The HotSpot JVM does release memory back to the OS, but does so reluctantly since resizing the heap is expensive and it is assumed that if you needed that heap once you'll need it again.

In general shrinking ability and behavior depends on the chosen garbage collector, the JVM version since shrinking capability was often introduced in later versions long after the GC itself was added. Some collectors may also require additional options to be passed to opt into shrinking. And some most likely never will support it, e.g. EpsilonGC. So if heap shrinking is desired it should be tested for a particular JVM version and GC configuration.

JDK 8 and earlier

There are no explicit options for prompt memory reclamation in these versions but you can make the GC more aggressive in general by setting -XX:GCTimeRatio=19 -XX:MinHeapFreeRatio=20 -XX:MaxHeapFreeRatio=30 which will allow it to spend more CPU time on collecting and constrain the amount of allocated-but-unused heap memory after a GC cycle.

If you're using a concurrent collector you can also set -XX:InitiatingHeapOccupancyPercent=N with N to some low value to let the GC run concurrent collections almost continuously, which will consume even more CPU cycles but shrink the heap sooner. This generally is not a good idea, but on some types of machines with lots of spare CPU cores but short on memory it can make sense.

If you're using G1GC note that it only gained the ability to yield back unused chunks in the middle of the heap with jdk8u20, earlier versions were only able to return chunks at the end of the heap which put significant limits on how much could be reclaimed.

If you're using a collector with a default pause time goal (e.g. CMS or G1) you can also relax that goal to place fewer constraints on the collector, or you can switch go the parallel collector to prioritize footprint over pause times.

To verify that shrinking occurs or to diagnose why a GC decides not to shrink you can use GC Logging with -XX:+PrintAdaptiveSizePolicy may also provide insight, e.g. when the JVM tries to use more memory for the young generation to meet some goals.

JDK 9

Added the -XX:-ShrinkHeapInSteps option which can be be used to apply the shrinking caused by the options mentioned in the previous section more aggressively. Relevant OpenJDK bug.

For logging -XX:+PrintAdaptiveSizePolicy has been replaced with -Xlog:gc+ergo

JDK 12

Introduced the option to enable prompt memory release for G1GC via the G1PeriodicGCInterval (JEP 346), again at the expense of some additional CPU. The JEP also mentions similar features in Shenandoah and the OpenJ9 VM.

JDK 13

Adds similar behavior for ZGC, in this case it is enabled by default.