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JVM OPTIONS

Cheatsheet for JVM flags that matter in production. For what each memory area is, see memory_areas.

Heap Memory

Flag	Purpose	Rule of Thumb	Risk if Wrong
-Xmx	Max heap size	RAM ≤16 GB → 50%; RAM >16 GB → 65%	Too small → frequent GC; too large → starves OS
-Xmx	Upper bound	Always ≤75% of RAM	OS and NIO buffers starved
-Xms	Initial heap	Set equal to -Xmx	Dynamic resizing → STW pauses

Direct Memory and Buffers

Flag	Purpose	Rule of Thumb	Risk if Wrong
-XX:MaxDirectMemorySize	Off-heap NIO/Netty cap	~15% of -Xmx, min 256 MB, max 25% of -Xmx	Too high → heap starved; too low → NIO fails
--add-opens=java.base/jdk.internal.misc=ALL-UNNAMED	Opens APIs for monitoring tools	Required for accurate direct memory metrics	Some monitoring tools break

OOM Troubleshooting

Flag	Purpose	Rule of Thumb
-XX:+HeapDumpOnOutOfMemoryError	Dump heap on OOM	Always enable
-XX:HeapDumpPath=	Dump location	Persistent storage: /hostdata/crash/{svc}.oom.hprof
-XX:ErrorFile=	JVM fatal error log	Persistent storage: /hostdata/crash/{svc}.hs_err_pid%p.log

DANGER

Wrong paths → dumps lost after container restart. Always use persistent volumes.

GC Logging

Flag	Purpose	Rule of Thumb
-Xlog:gc*	GC log output	Logs synced: filecount=1, filesize=200m. Not synced: filecount=5, filesize=200m

Logging and Encoding

Flag	Purpose	Rule of Thumb
-Dlog4j2.contextSelector	Async logging	Set to org.apache.logging.log4j.core.async.AsyncLoggerContextSelector
-Dfile.encoding	Default encoding	UTF-8

Security and Stability

Flag	Purpose
-Dfastjson.parser.safeMode=true	Blocks Fastjson deserialization RCE
-Djava.awt.headless=true	Disables GUI rendering (saves CPU)
-Dspring.devtools.restart.enabled=false	Prevents hot-reload in production

Profiling

Flag	Purpose
-XX:+UseZingMXBeans	Exposes JVM metrics
-XX:+ProfileLiveObjects	Enables jmap -histo object histograms
-XX:ProfileLogIn / -XX:ProfileLogOut	JIT profile log paths for ReadyNow warmup

Quick Rules

Heap

• Pin size: -Xms = -Xmx
• RAM ≤16 GB → 50%. RAM >16 GB → 65%.

Direct memory

• Default ~15% of -Xmx, always ≥256 MB, ≤25% of -Xmx

Logging

• Async logging always. GC logs rotated.

Troubleshooting

• Heap dumps + error logs on persistent volumes.

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References

• memory_areas
• memory_diagnostics