JEP 404: Generational Shenandoah (Experimental)

AuthorsBernd Mathiske, Kelvin Nilsen, William Kemper, and Ramki Ramakrishna
OwnerRoman Kennke
Componenthotspot / gc
Discussionhotspot dash gc dash dev at openjdk dot java dot net
Reviewed byAleksey Shipilev, Roman Kennke
Endorsed byVladimir Kozlov
Created2021/02/01 22:49
Updated2023/08/02 13:02


Enhance the Shenandoah garbage collector with experimental generational collection capabilities to improve sustainable throughput, load-spike resilience, and memory utilization.


The main goal is to provide an experimental generational mode, without breaking non-generational Shenandoah, with the intent to make it the default in a future release.

Other goals are set relative to non-generational Shenandoah:


Success Metrics


Garbage collectors with concurrent compaction are capable of completely blending GC pause times into the single-digit millisecond range of other common JVM pauses, while also leaving mutator execution speed nearly unfettered. Non-generational Shenandoah garbage collection already provides this ideal GC behavior for latency-sensitive Java applications. However, it can only achieve this within limited operational envelopes (i.e., combinations of heap occupancy and allocation rate).

A classic approach to minimize the average GC cost is to adopt the generational hypothesis that most objects die young, and concentrate cycles on dealing with young and therefore mostly dead objects. Compared to the generational collectors G1, CMS, and Parallel, non-generational Shenandoah tends to require more heap headroom and work harder to recover space occupied by unreachable objects.

Region-based generational collectors are capable of dynamically adapting their generation sizes and copying policy in response to changes in object demographics, allowing the collector to adjust for workloads that do not honor the generational hypothesis. Even when surviving objects are copied in the young generation more often than would be necessary, this cost is often dwarfed by the reduced frequency of marking long-lived objects in comparison with non-generational collectors.

A concurrent collector that is also generational and can dynamically adjust its young generation’s size and related operational parameters can both achieve low pause times and stay competitive in other performance aspects.


This enhancement of the Shenandoah garbage collector separates the Java heap into two generations. As in other generational collectors, GC efforts focus on the young generation, i.e., the one in which allocations by the mutator occur and where ephemeral objects can be reclaimed with reduced effort. We propose the following approach for an initial implementation.

The collection algorithms operating on each generation are closely based on non-generational Shenandoah. Within the young generation, Generational Shenandoah uses the same heuristics as traditional Shenandoah to distinguish areas of memory that hold newly allocated objects from areas of memory holding objects that survived one or more recent young-generation collections.

Each generation is formed by a subset of the Shenandoah heap’s regions. At any given time, a region is considered either free or dedicated to either the young or the old generation. The size of each generation is given by its occupied regions plus a quota of free regions. Overreach into the free quota of the respective other generation is tolerated, but it accelerates collection triggering and can lead to degenerated and full collections. We are actively refining the algorithms to control collection-phase scheduling, young-generation sizing, tenuring age, and other auto-tuning mechanisms.

Shenandoah has a unique Load Reference Barrier (LRB) that supports 32-bit builds and compressed 32-bit object pointers (“compressed oops”) in 64-bit builds. To constrain impact on the mutator we use this same LRB for both generations, without any changes, and use a single evacuator for both old and young collection efforts. Typical evacuation phases collect garbage either exclusively from young regions or from a combination of young and old regions. This behavior mimics G1’s young and mixed collections. The principal improvement over G1 is that Generational Shenandoah’s young and mixed collections are concurrent with the mutator.

The generation-specific marking phases are largely decoupled from each other. Concurrent old-generation marking proceeds in the background during the time that young-generation marking and evacuation occurs multiple times. Old-generation marking can be preempted to execute higher priority young-generation collections. Once old-generation marking completes, subsequent evacuations and reference updates include old-generation regions until the entire old-generation collection set has been processed.

For the remembered-set implementation, we use existing card marking code as borrowed from the Parallel and CMS GC implementations and supplement this with new code that allows remembered set scanning to run concurrently with mutator execution.

Non-generational Shenandoah’s existing SATB barriers are generalized to serve the combined needs of young-generation and old-generation concurrent marking. The post-processing of SATB buffers treats references to old-generation memory differently than references to young-generation memory, but the fast path through these barriers remains unchanged.

Development is being conducted in the openjdk/shenandoah repository on the master branch.


The new generational feature is part of the Shenandoah code base, but it has no runtime effect unless it is activated by the JVM command line options

-XX:+UnlockExperimentalVMOptions -XX:ShenandoahGCMode=generational

in which case Shenandoah will use its generational mode.

The project wiki will provide details on how to configure and tune the JVM for effective generational-mode operation of applications running with Shenandoah GC.


Azul Systems’ C4 collector is already generational, but not available in open source. A generational mode of ZGC is also under development. Neither of these options supports compressed object pointers. However, the vast majority of Java heaps that we see (e.g., in cloud services) are well below 32 GB in size and thus able to take advantage of this space-saving and performance-improving feature.


Most existing functional and stress tests are collector-agnostic and can be reused as-is. We will integrate additional test run configurations for the new generational mode along with new mode-specific functional, performance, and stress tests.

The current focus of performance optimization is on x86 and AArch64 with Linux. SAP has ported generational mode to PowerPC and tested on that platform. We run CI tests on Linux, macOS, and Windows. Support for generational mode for other platforms can be implemented and optimized later.

Risks and Assumptions