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Generational ZGC vs G1 on JDK 25: benchmarks, internals and captured results

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Complete companion suite for the ankurm.com guide. Everything was compiled and
executed on java 25.0.3+9-LTS-195 (AMD Ryzen 5 5600U, Windows 11); every file
under results/ is unedited program output.

Code
  latency/    open-loop latency harness that measures from intended arrival, so
              coordinated omission is accounted for rather than hidden. Reports
              response time and service time side by side; the gap reached 2910x
              on G1.
  jmh/        four microbenchmarks isolating one mechanism each: TLAB allocation,
              the ZGC load barrier (with a primitive-load control), the write
              barrier (with null / old-to-old / primitive controls), and promotion
              pressure.
  tuning/     provokes ZGC allocation stalls and reads its own JFR recording back,
              grouped by page class. jdk.ZAllocationStall is enabled without a
              threshold, because the 10 ms default hides most stalls.
  internals/  ZGC page classes vs G1 humongous, read from the live VM via
              HotSpotDiagnosticMXBean and jdk.ZPageAllocation events.
  analysis/   unified GC log parser that keeps stop-the-world pauses and
              concurrent phases in separate buckets.
  env/        environment capture; proves generational mode from JMX bean names.

Docs
  Eight chapters covering the JEP 439/474/490 timeline, colored pointers and both
  barriers, allocation stalls, a full flag reference, logging and JFR, benchmark
  methodology, corner cases, and a decision procedure.

Headline result (60 s, 20k req/s, 2 GB heap, ~585 MB live)
  p50               ZGC 0.006 ms   G1 0.005 ms
  p99.9             ZGC 1.437 ms   G1 95.169 ms
  total STW time    ZGC 1.503 ms   G1 1,139.624 ms
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Ankur
2026-07-31 08:47:16 +05:30
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results/05-pages-g1.txt Normal file
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==========================================================================
Page / region geometry as reported by THIS JVM
==========================================================================
max heap : 2,048 MB
UseG1GC / UseZGC : true / false
G1HeapRegionSize : 1,048,576 bytes (1 MB)
=> G1 humongous threshold: objects larger than 524,288 bytes (512 KB) go straight to old gen
No jdk.ZPageAllocation events (expected under G1).
Objects allocated in this run
65,536 bytes (64 KB) ordinary G1 allocation in an eden region
262,144 bytes (256 KB) ordinary G1 allocation in an eden region
524,288 bytes (512 KB) ordinary G1 allocation in an eden region
2,097,152 bytes (2 MB) HUMONGOUS on G1 -- allocated directly in old gen, spans whole regions
4,194,304 bytes (4 MB) HUMONGOUS on G1 -- allocated directly in old gen, spans whole regions
16,777,216 bytes (16 MB) HUMONGOUS on G1 -- allocated directly in old gen, spans whole regions
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