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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
This commit is contained in:
Ankur
2026-07-31 08:47:16 +05:30
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==========================================================================
Page / region geometry as reported by THIS JVM
==========================================================================
max heap : 2,048 MB
UseG1GC / UseZGC : false / true
G1HeapRegionSize : 0 (ergonomic / not applicable)
=> ZGC small pages are 2 MB; objects up to 1/8 of a page (256 KB) use them.
Larger objects use a medium page, and anything bigger gets a Large page
of its own -- which ZGC frees rather than relocates.
ZGC internals flags visible in this JVM
ZFragmentationLimit = 5.0
ZCollectionIntervalMinor = -1.0
ZCollectionIntervalMajor = -1.0
ZAllocationSpikeTolerance = 2.0
ZUncommit = false
ZUncommitDelay = 300
SoftMaxHeapSize = 2,147,483,648 bytes (2048 MB)
ZGC pages actually allocated while creating the objects above
page type count total bytes
Large 2 25,165,824
Medium 1 33,554,432
Objects allocated in this run
65,536 bytes (64 KB) expect ZGC Small page (2 MB, shared with other objects)
262,144 bytes (256 KB) expect ZGC Medium page (shared; size is a range in JDK 25)
524,288 bytes (512 KB) expect ZGC Medium page (shared; size is a range in JDK 25)
2,097,152 bytes (2 MB) expect ZGC Medium page (shared; size is a range in JDK 25)
4,194,304 bytes (4 MB) expect ZGC Large page -- one page for this object, freed not relocated
16,777,216 bytes (16 MB) expect ZGC Large page -- one page for this object, freed not relocated
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