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-g1.txt

@@ -0,0 +1,40 @@
+==============================================================================
+GC log analysis: g1-latency.log
+==============================================================================
+lines parsed            : 1,257
+young / minor cycles    : 94
+old / major cycles      : 0
+ZGC allocation stalls   : 0
+G1 evacuation failures  : 16
+full GCs                : 4
+
+STOP-THE-WORLD PAUSES (this is application freeze time)
+  count        : 68
+  total        : 1,139.624 ms
+  mean         : 16.759 ms
+  p50          : 12.857 ms
+  p99          : 104.449 ms
+  max          : 104.449 ms
+
+  by phase
+    Pause Young (Normal)         n=17     total= 403.699 ms  max=103.308 ms
+    Pause Young (Concurrent Start) n=11     total= 153.384 ms  max= 20.587 ms
+    Pause Remark                 n=10     total=  13.736 ms  max=  1.745 ms
+    Pause Cleanup                n=9      total=   2.472 ms  max=  0.438 ms
+    Pause Young (Prepare Mixed)  n=8      total= 110.491 ms  max= 24.639 ms
+    Pause Young (Mixed)          n=11     total= 247.284 ms  max= 30.879 ms
+    Pause Full                   n=2      total= 208.558 ms  max=104.449 ms
+
+CONCURRENT PHASES (application keeps running -- NOT freeze time)
+    Concurrent Scan Root Regions n=11     total=  12.317 ms  max=  1.867 ms
+    Concurrent Mark From Roots   n=11     total= 191.931 ms  max=102.516 ms
+    Concurrent Preclean          n=10     total=   0.265 ms  max=  0.050 ms
+    Concurrent Mark              n=10     total= 106.217 ms  max= 14.524 ms
+    Concurrent Rebuild Remembered Sets and Scrub Regions n=10     total= 146.586 ms  max=113.784 ms
+    Concurrent Clear Claimed Marks n=9      total=   0.227 ms  max=  0.039 ms
+    Concurrent Cleanup for Next Mark n=9      total=  31.227 ms  max=  4.862 ms
+    Concurrent Mark Cycle        n=11     total= 406.062 ms  max=125.183 ms
+
+  Adding the block above to the pause block would be a category error:
+  concurrent time overlaps application execution by design.
+