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

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

results/03-latency-zgc.txt

@@ -0,0 +1,48 @@
+==============================================================================
+ZGC / G1 open-loop latency harness
+==============================================================================
+collector : ZGC Minor Cycles + ZGC Minor Pauses + ZGC Major Cycles + ZGC Major Pauses
+java      : 25.0.3+9-LTS-195
+threads=4 rate=20,000/s warmup=20s measure=60s
+liveSet=150,000 entries x 4,096 B (~585 MB live)
+garbage=16,384 B/request  reads=64/request  mutationProbability=0.20
+nominal allocation rate ~328 MB/s   seed=20260730
+------------------------------------------------------------------------------
+building live set ... 150,000 entries, ~589 MB live, took 310 ms
+warming up for 20 s at 20,000 req/s ...
+measuring for 60 s at 20,000 req/s ...
+
+------------------------------------------------------------------------------
+achieved throughput : 19,973 req/s over 60.1 s
+
+RESPONSE TIME  (from intended arrival -- the honest number)  (n=1,200,000)
+  mean       0.016 ms
+  p50        0.006 ms
+  p90        0.009 ms
+  p99        0.029 ms
+  p99.9      1.437 ms
+  p99.99    23.675 ms
+  max       32.178 ms
+
+SERVICE TIME   (from actual start -- hides queueing)  (n=1,200,000)
+  mean       0.007 ms
+  p50        0.006 ms
+  p90        0.009 ms
+  p99        0.025 ms
+  p99.9      0.143 ms
+  p99.99     0.269 ms
+  max        8.353 ms
+
+coordinated-omission gap at p99.9 : 1.437 ms response vs 0.143 ms service (10.0x)
+
+GC activity during the measured window
+  ZGC Minor Cycles       collections=18     time=    919 ms   [cycle (mostly concurrent)]
+  ZGC Minor Pauses       collections=54     time=      0 ms   [stop-the-world]
+  ZGC Major Cycles       collections=3      time=    667 ms   [cycle (mostly concurrent)]
+  ZGC Major Pauses       collections=15     time=      0 ms   [stop-the-world]
+
+CSV (response time)
+collector,n,mean_ms,p50_ms,p90_ms,p99_ms,p999_ms,p9999_ms,max_ms
+ZGC Minor Cycles + ZGC Minor Pauses + ZGC Major Cycles + ZGC Major Pauses,1200000,0.016,0.006,0.009,0.029,1.437,23.675,32.178
+
+liveSet checksum: 1