# Generational ZGC vs G1 on JDK 25 — runnable benchmarks and internals Companion repository for **[Generational ZGC on JDK 25: Benchmarks vs G1](https://ankurm.com/generational-zgc-jdk-25-vs-g1/)** on [ankurm.com](https://ankurm.com). The blog post covers the concepts most people need. **This repository covers everything** — every flag, every failure mode, every measurement pitfall, with runnable code, real captured output, and the reasoning behind each choice. Everything here was compiled and executed on `java 25.0.3+9-LTS-195`. Raw, unedited output for every run is in [`results/`](results). --- ## Quick start ```console git clone https://ankurm.com/git.app/asmhatre/zgc-jdk25-benchmarks.git cd zgc-jdk25-benchmarks # Windows pwsh scripts/run-all.ps1 -JavaHome 'C:\Program Files\Java\jdk-25.0.3' # Linux / macOS ./scripts/run-all.sh /path/to/jdk-25 ``` Or run a single piece — nothing but a JDK 25 is required for anything outside `jmh/`: ```console javac -d out $(find latency tuning internals analysis -name '*.java') java -XX:+UseZGC -Xms2g -Xmx2g -cp out com.ankurm.zgc.latency.LatencyHarness java -XX:+UseG1GC -Xms2g -Xmx2g -cp out com.ankurm.zgc.latency.LatencyHarness ``` --- ## Headline result 60 seconds, 20,000 req/s open loop, 2 GB heap, ~585 MB live set, ~328 MB/s allocation rate. Full output: [`results/03-latency-zgc.txt`](results/03-latency-zgc.txt), [`results/03-latency-g1.txt`](results/03-latency-g1.txt). | Metric | Generational ZGC | G1 | |---|---|---| | p50 response time | 0.006 ms | **0.005 ms** | | p90 | 0.009 ms | 0.009 ms | | p99 | **0.029 ms** | 8.042 ms | | p99.9 | **1.437 ms** | 95.169 ms | | p99.99 | **23.675 ms** | 126.949 ms | | max | **32.178 ms** | 133.018 ms | | total stop-the-world time | **1.503 ms** | 1,139.624 ms | | longest single pause | **0.054 ms** | 104.449 ms | | Full GCs / evacuation failures | **0 / 0** | 4 / 16 | G1 wins the median. ZGC wins everything past p99. The two are indistinguishable up to p90 — which is the part most "ZGC is faster" posts leave out. --- ## What is in here ### Code | Module | What it demonstrates | Deps | |---|---|---| | [`env/Env.java`](env/Env.java) | Environment capture; proves generational mode from JMX bean names | none | | [`latency/`](latency/src/com/ankurm/zgc/latency) | **Open-loop** latency harness with coordinated-omission accounting | none | | [`tuning/AllocationStallDemo.java`](tuning/src/com/ankurm/zgc/tuning/AllocationStallDemo.java) | Provokes and measures ZGC allocation stalls via JFR | none | | [`internals/PageSizeDemo.java`](internals/src/com/ankurm/zgc/internals/PageSizeDemo.java) | ZGC page classes vs G1 humongous, read from the live VM | none | | [`analysis/GcLogParser.java`](analysis/src/com/ankurm/zgc/analysis/GcLogParser.java) | Unified-log parser that keeps pauses and concurrent phases apart | none | | [`jmh/`](jmh/src/main/java/com/ankurm/zgc/jmh) | Throughput and barrier microbenchmarks | JMH 1.37 | ### JMH benchmarks | Class | Isolates | |---|---| | [`AllocationBench`](jmh/src/main/java/com/ankurm/zgc/jmh/AllocationBench.java) | TLAB allocation path at 64 B / 1 KB / 32 KB | | [`LoadBarrierBench`](jmh/src/main/java/com/ankurm/zgc/jmh/LoadBarrierBench.java) | **ZGC's load barrier** — pointer chase vs primitive sum control | | [`StoreBarrierBench`](jmh/src/main/java/com/ankurm/zgc/jmh/StoreBarrierBench.java) | **Write barriers** — old→young vs null vs old→old vs primitive control | | [`PromotionBench`](jmh/src/main/java/com/ankurm/zgc/jmh/PromotionBench.java) | Medium-lived objects; the case where "most objects die young" fails | ### Documentation | Chapter | Covers | |---|---| | [1. The generational model](docs/01-generational-model.md) | JEP 439 → 474 → 490 timeline, what `ZGenerational` does now, why there is no `-Xmn` | | [2. Barriers](docs/02-barriers.md) | Colored pointers, load barrier, store barrier, remembered sets, `ZBufferStoreBarriers` | | [3. Allocation stalls](docs/03-allocation-stalls.md) | **The ZGC failure mode pause charts cannot see**, and the 10 ms JFR threshold that hides it | | [4. Tuning reference](docs/04-tuning-reference.md) | Every ZGC product + diagnostic flag, and the G1 flags that still matter | | [5. Logging and JFR](docs/05-logging-and-jfr.md) | Reading `-Xlog:gc*`, the JMX bean trap, the JFR events worth enabling | | [6. Methodology](docs/06-methodology.md) | Coordinated omission, timer granularity, the JMH `-jvmArgs` fork trap | | [7. Corner cases](docs/07-corner-cases.md) | Compressed oops, large-page fragmentation, virtual thread stacks, containers, `System.gc()` | | [8. Choosing a collector](docs/08-choosing.md) | A decision procedure, sizing guidance, when the answer is neither | --- ## Five findings you will not read elsewhere 1. **G1's coordinated-omission gap on this workload is 2910x.** A closed-loop harness reports G1's p99.9 as 0.033 ms. It is really 95.169 ms. → [docs/06-methodology.md §6.1](docs/06-methodology.md) 2. **ZGC disables compressed oops.** Every reference is 8 bytes instead of 4, so a reference-dense heap can grow 30–40% on switching collector, before any latency benefit. → [docs/07-corner-cases.md §7.1](docs/07-corner-cases.md) 3. **`jdk.ZAllocationStall` has a 10 ms default JFR threshold.** In a run whose mean stall was 3.98 ms, a default profile discards most of them. → [docs/03-allocation-stalls.md §3.3](docs/03-allocation-stalls.md) 4. **`new byte[4*1024*1024]` gets a ZGC Large page, not a Medium one.** The 16-byte array header pushes it over the medium-object limit, and it consumes an 8 MB page. → [docs/07-corner-cases.md §7.2](docs/07-corner-cases.md) 5. **`SoftMaxHeapSize` and `ZAllocationSpikeTolerance` did nothing under sustained overload.** Measured: 32,200 / 31,967 / 32,442 stalls across baseline and both mitigations. They buy headroom for *spikes*; they cannot conjure heap that does not exist. → [results/04-stalls-zgc-softmax.txt](results/04-stalls-zgc-softmax.txt) --- ## Reference machine | | | |---|---| | CPU | AMD Ryzen 5 5600U, 6 cores / 12 threads | | RAM | 15.3 GB | | OS | Windows 11 Home | | JDK | `java 25.0.3+9-LTS-195` (Oracle) | | Heap | `-Xms2g -Xmx2g` for all latency runs; `-Xms512m -Xmx512m` for stall runs | A laptop, not a server. The absolute numbers are illustrative; the *relationships* are what generalise. Run it on your own hardware — that is what it is for. --- ## Licence MIT. See [LICENSE](LICENSE).