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G1 GC is introduced in Java 8 and made default Garbage Collector in Java 9. So What happened to Concurrent Mark Sweep (CMS) Garbage Collector? Why is CMS out?

In case you have missed changes in Java 7, 8 and 9 - look here

A note from OpenJDK on depreciation of CMS

"Dropping support for CMS and then removing the CMS code, or at least more thoroughly segregating it, will reduce the maintenance burden of the GC code base and accelerate new development. The G1 garbage collector is intended, in the long term, to be a replacement for most uses of CMS."

Concurrent Mark Sweep (CMS) Garbage Collector

Earlier, The parallel garbage collectors in Hotspot are designed to minimise the amount of time that the application spends undertaking garbage collection, which is termed throughput. This isn't an appropriate tradeoff for all applications - some require individual pauses to be short as well, which is known as a latency requirement.

The Concurrent Mark Sweep (CMS) collector is designed to be a lower latency collector than the parallel collectors. The key part of this design is trying to do part of the garbage collection at the same time as the application is running. This means that when the collector needs to pause the application's execution it doesn't need to pause for as long.

CMS uses the parallel stop-the-world mark-copy algorithm in the Young Generation and concurrent mark-sweep algorithm in the Old Generation.

Young Generation - Par New Collector - mark-copy Algorithm

Old Generation - CMS Collector - mark-sweep Algorithm

Phases of CMS

Performance Issues with CMS

1. Concurrent Mode Failure
2. Excessive GC Time and OutOfMemoryError
3. Floating Garbage
4. Multiple Pauses in GC cycle

Floating garbage

This is really a performance hit. Have u missed Compaction phase in CMS? Yes, it is not in CMS. So after many GC cycles, Heap becomes fragmented over a period of time. See in screens below

This is state of Old Generation before collection. And below is state of Old Gen after CMS run.

Free space in CMS is maintained as Linked List. Promotions, new memory allocations in Linked List costs more than just a pointer change, not in O(1) time. So all this makes new object memory allocation time-taking. Larger heap sizes are required by CMS collector to overcome floating garbage problem and perform well.

Quoting from OpenJDK posts,

Requests were made for a CMS maintainer, but "so far, no one has stepped up." He concludes that post, "In any case, Oracle does intend to stop maintaining CMS at some point in the not-to-distant future, and if no one ever steps up then we'll remove the code."

So it is bye bye to CMS which made way for G1 Garbage Collector in Java 9.

#jvm #jmm #garbagecollection #gc #java #java8 #java9 #performance #google #oracle

Bringing all this to you from multiple sources. Thanks to you and sources.

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