Native Image Committer and Community Meeting 2026-03-26 #13201

wirthi · 2026-03-25T09:58:42Z

wirthi
Mar 25, 2026
Collaborator

List of all past and upcoming meetings: #3933

Note that we have the meeting at 17:00 CET; due to the US being on DST already, while the EU is not yet, the meeting time will be an hour later than usual for the US audience. https://www.timeanddate.com/worldclock/fixedtime.html?iso=20260326T09&p1=224

The meeting after this one is planned for April 30th, 2026. The meeting will typically be on the last Thursday of each month.

PRs listed below were closed between February 13th, 2026 and March 25th, 2026.

New and Noteworthy

Questions

GraalVM Deep Dives: Peter Hofer: Compacting Old Gen and other news on the Serial GC

#12913: [GR-52538] [GR-69523] [GR-73129] New GC policy and enable CompactingOldGen by default.
#13088: [GR-71279] Sweep chunks with pinned objects in copying collections.

The old generation now uses mark-compact collection by default, CompactingOldGen.
We introduced the Adaptive2 collection policy as default to better fit mark-compact collection.
Pinned objects are handled better.

`CompactingOldGen`

Before this change, a full GC collected the old generation by copying. This requires extra memory, up to 2x the current heap size (and in extreme cases 2x the maximum heap size).

CompactingOldGen uses marking and in-place compaction for full collections as follows:

Absorb the young generation into the old generation (as before).
Mark objects reachable from roots.
Plan compaction: assign new locations to entire sequences of surviving objects.
Fixup: adjust references to point to the new object locations.
Compact: move sequences of surviving objects to their new locations.

One detail worth noting is that move information is stored in gaps between sequences of live objects, rather than in object headers or side tables. This avoids using extra memory, but requires a separate planning and fixup pass. It also means that when we need an object's new location, we need to identify the object sequence it is part of and find its move information. We temporarily build a lookup table for that in the card table memory.

Code point of interest: https://github.com/oracle/graal/blob/master/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/CompactingOldGeneration.java

`Adaptive2` collection policy

Because the previous default collection policy did not fit CompactingOldGen as well, we also introduced a new default policy: Adaptive2. It is less complex and avoids some erratic behaviors we saw with the previous policy.

Most importantly, its inputs are:

Throughput, the ratio of mutator time to GC time.
The time between collections.
Young-generation pressure: survivor overflow, promoted bytes, and promotion failures.

At a high level, Adaptive2 does three important things:

Grow the eden space when young collections are too frequent.
Trigger a full collection when promotion pressure suggests that the next young collection would not fit into the old generation.
Prefer less promotion and more young-generation collection when full collections are comparatively too expensive.

The policy is based on HotSpot's ParallelGC policy (also recently re-engineered) with different parameters, most importantly:

GC_TIME_RATIO = 1:
this corresponds to a throughput goal of 50%, which is intentionally low. (SerialGC is single-threaded, so it cannot reach very high throughput goals for highly multi-threaded applications.)
MIN_GC_DISTANCE_SECOND = 0.020:
once the throughput goal above is met, this gives the application enough time to run and to avoid too much overhead from frequent collections.

Code point of interest: https://github.com/oracle/graal/blob/master/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/AdaptiveCollectionPolicy2.java

Benchmark results with `CompactingOldGen` and `Adaptive2`

From the benchmark runs used for this change, on Community Edition:

On our microservice benchmarks,
- throughput is around 4% higher (geometric mean).
- peak memory usage is around 12% lower (geometric mean).
- higher-percentile latencies also improved.
Across all our benchmarks,
- peak memory usage is reduced by around 20% (geometric mean)
- execution time and throughput remain similar to before.

Falling back to the previous behavior

Minor regressions are difficult to avoid with a change in behavior like this, but please let us know if you encounter a serious regression.

Copying collections with the old policy can be restored with:

-H:-CompactingOldGen -H:InitialCollectionPolicy=Adaptive

Pinning improvements in copying collections

The heap consists of fixed-size chunks. If a chunk contains a pinned object, that restricts how the collector can collect the chunk.

Previously, copying collections effectively treated chunks with pinned objects as if all their objects were alive and pinned. This could also keep objects in other chunks alive.

Full collections with CompactingOldGen handle such chunks better by marking live objects in them and sweeping the chunk.

We applied the same idea to copying collections, so that unpinned live objects are copied out and the chunk is then swept so that only pinned objects remain. This also applies to young generation collections with CompactingOldGen.

Code point of interest: https://github.com/oracle/graal/blob/master/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/SweepAndPromotePinnedChunkVisitor.java

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Native Image Committer and Community Meeting 2026-03-26 #13201

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Native Image Committer and Community Meeting 2026-03-26 #13201

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wirthi Mar 25, 2026 Collaborator

New and Noteworthy

Questions

GraalVM Deep Dives: Peter Hofer: Compacting Old Gen and other news on the Serial GC

CompactingOldGen

Adaptive2 collection policy

Benchmark results with CompactingOldGen and Adaptive2

Falling back to the previous behavior

Pinning improvements in copying collections

Replies: 0 comments

wirthi
Mar 25, 2026
Collaborator

`CompactingOldGen`

`Adaptive2` collection policy

Benchmark results with `CompactingOldGen` and `Adaptive2`