Java 27 and 26: runnable demos and captured output for every JEP, plus version lanes

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3

scripts/g1-tuning.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+# Why does G1 on 1 CPU touch ~490 MB in a 0.5 s process? Two suspects, tested:
+#   1. the JDK 27 change of MinHeapFreeRatio/MaxHeapFreeRatio from 40/70 to 0/100   -> restored here; no effect
+#   2. G1 simply being allowed to grow toward the 512 MB default maximum heap        -> capped here; RSS follows the cap
+#   ./scripts/g1-tuning.sh   -> docs/output/06-g1-tuning-1cpu.txt
+set -euo pipefail
+source "$(dirname "$0")/env.sh"
+BUILD="$ROOT/build/g1"; mkdir -p "$BUILD"
+"$JDK27/bin/javac" --release 26 -d "$BUILD" "$ROOT"/g1-container/src/*.java
+{
+  echo "# JDK 27, G1 (the default), 1 CPU / 2 GB container, 400 batches, 2 runs per configuration"
+  echo
+  echo "=== flags that differ between 26 and 27 (from -XX:+PrintFlagsFinal)"
+  for img in "$IMG26" "$IMG27"; do
+    echo "--- $img"
+    docker run --rm --cpus=1 --memory=2g "$img" java -XX:+PrintFlagsFinal -version 2>/dev/null \
+      | grep -E ' (MinHeapFreeRatio|MaxHeapFreeRatio|MaxHeapSize|ConcGCThreads|ParallelGCThreads|UseCompactObjectHeaders|G1IHOP|UseCompressedClassPointers) ' || true
+  done
+  echo
+  for flags in "" "-XX:MinHeapFreeRatio=40 -XX:MaxHeapFreeRatio=70" "-Xmx256m"; do
+    echo "=== $IMG27  flags: [${flags:-none}]"
+    for i in 1 2; do
+      docker run --rm --cpus=1 --memory=2g -v "$BUILD:/app:ro" "$IMG27" java $flags -cp /app Workload 400 \
+        | grep -E 'wall|gc count|peak'
+    done
+  done
+} > "$OUT/06-g1-tuning-1cpu.txt" 2>&1
+cat "$OUT/06-g1-tuning-1cpu.txt"