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/06-gclog-zgc.txt

@@ -0,0 +1,36 @@
+==============================================================================
+GC log analysis: zgc-latency.log
+==============================================================================
+lines parsed            : 2,247
+young / minor cycles    : 45
+old / major cycles      : 17
+ZGC allocation stalls   : 40
+G1 evacuation failures  : 0
+full GCs                : 0
+
+STOP-THE-WORLD PAUSES (this is application freeze time)
+  count        : 106
+  total        : 1.503 ms
+  mean         : 0.014 ms
+  p50          : 0.014 ms
+  p99          : 0.044 ms
+  max          : 0.054 ms
+
+  by phase
+    Pause Mark Start (Major)     n=8      total=   0.139 ms  max=  0.021 ms
+    Pause Mark End               n=38     total=   0.683 ms  max=  0.044 ms
+    Pause Relocate Start         n=38     total=   0.330 ms  max=  0.021 ms
+    Pause Mark Start             n=22     total=   0.351 ms  max=  0.054 ms
+
+CONCURRENT PHASES (application keeps running -- NOT freeze time)
+    Concurrent Mark              n=38     total= 894.499 ms  max= 78.403 ms
+    Concurrent Mark Free         n=38     total=   0.097 ms  max=  0.033 ms
+    Concurrent Reset Relocation Set n=38     total=   4.331 ms  max=  0.934 ms
+    Concurrent Select Relocation Set n=38     total=  48.991 ms  max=  5.372 ms
+    Concurrent Relocate          n=38     total=1092.392 ms  max=151.380 ms
+    Concurrent Process Non       n=8      total=   5.635 ms  max=  0.984 ms
+    Concurrent Remap Roots       n=8      total=  10.919 ms  max=  2.207 ms
+
+  Adding the block above to the pause block would be a category error:
+  concurrent time overlaps application execution by design.
+