ColdFusion JVM Tuning: How Much RAM to Allocate and Which G1GC Flags Actually Work

ColdFusion’s jvm.config still ships with -XX:+UseParallelGC as the default garbage collector — not G1GC — even on ColdFusion 2025 running JDK 21 LTS. This is verified across multiple Adobe and community sources. If you don't explicitly change it, your CF server is using the throughput-optimized Parallel collector, not the low-pause G1 collector that most modern Java tuning guides assume. For heap sizing: per cfguide.io's CF 2025 documentation, allocate 50-70% of system RAM for the JVM heap, with 4-8GB as a starting point for production that you adjust based on actual usage. Set -Xms equal to -Xmx to avoid resize churn. For G1GC: switch is justified when heap is larger than ~6GB or pause-time minimization matters more than throughput (per Charlie Arehart's published guidance). The flags that actually work in JDK 21 are -XX:+UseG1GC -XX:MaxGCPauseMillis=200 -XX:+UseStringDeduplication plus thread tuning. Flags to remove if present: -XX:+AggressiveOpts (removed in JDK 12), -XX:+UseConcMarkSweepGC (removed in JDK 14), -XX:PermSize / -XX:MaxPermSize (replaced by Metaspace in Java 8), -XX:+UseCompressedOops (default in JDK 7+, no need to specify). For very large heaps in JDK 21: Generational ZGC (-XX:+UseZGC -XX:+ZGenerational) is now production-ready and may outperform G1GC on heaps over 32GB.

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