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JVM Tuning Parameters Guide

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JVM Parameter Categories

JVM parameters fall into three categories:

Type Format Description
Standard - Supported by all JVM implementations (e.g., -version, -cp)
Non-standard -X Not guaranteed on all JVMs but commonly available (e.g., -Xms, -Xmx)
Developer -XX Extension parameters specific to a JVM implementation (e.g., -XX:+UseG1GC)

Boolean-type developer parameters use +/- switches:

  • -XX:+UseG1GC enables
  • -XX:-UseG1GC disables

Non-boolean types use = assignment:

  • -XX:MaxHeapSize=4g

Memory Parameters

Heap Memory

Parameter Description Recommended Value
-Xms Initial heap size Same as -Xmx
-Xmx Maximum heap size 50-80% of physical memory
-Xmn Young generation size 30-40% of heap
-XX:NewRatio Old/young generation ratio 2 (default)
-XX:SurvivorRatio Eden/Survivor ratio 8 (default)
-XX:MaxTenuringThreshold Age threshold for promotion to old generation 15 (default)
-XX:PretenureSizeThreshold Size threshold for direct allocation in old generation No default

Best Practices

# Production environment recommendation
-Xms4g -Xmx4g        # Fixed heap size, avoid dynamic expansion/contraction
-Xmn1536m            # Young generation ~37.5% of heap
-XX:SurvivorRatio=8  # Eden:S0:S1 = 8:1:1
-XX:MaxTenuringThreshold=15

Why set -Xms and -Xmx to the same value?

  • Avoid performance jitter from heap resizing
  • Avoid Full GC during expansion
  • Pre-allocate memory to ensure availability

Non-Heap Memory

Parameter Description Recommended Value
-XX:MetaspaceSize Initial metaspace size 256m
-XX:MaxMetaspaceSize Maximum metaspace size 256m-512m
-Xss Per-thread stack size 256k-1m
-XX:MaxDirectMemorySize Maximum direct memory Same as -Xmx
-XX:ReservedCodeCacheSize JIT code cache size 240m (default) 
# Metaspace recommendation
-XX:MetaspaceSize=256m -XX:MaxMetaspaceSize=512m

# Thread stack
-Xss512k    # General applications
-Xss256k    # High-concurrency applications (saves memory with many threads)

GC Parameters

Collector Selection

Parameter Collector Combination
-XX:+UseSerialGC Serial + Serial Old
-XX:+UseParNewGC ParNew + Serial Old
-XX:+UseParallelGC Parallel Scavenge + Parallel Old
-XX:+UseConcMarkSweepGC ParNew + CMS + Serial Old (fallback)
-XX:+UseG1GC G1
-XX:+UseZGC ZGC
-XX:+UseShenandoahGC Shenandoah

Parallel Scavenge Parameters

Parameter Description Default
-XX:MaxGCPauseMillis Maximum GC pause target None
-XX:GCTimeRatio Throughput target 99
-XX:+UseAdaptiveSizePolicy Adaptive sizing Enabled
-XX:ParallelGCThreads GC thread count CPU cores

CMS Parameters

Parameter Description Default
-XX:CMSInitiatingOccupancyFraction Old generation occupancy trigger threshold 68 (first time)/92
-XX:+UseCMSCompactAtFullCollection Compact fragmentation after Full GC Enabled
-XX:CMSFullGCsBeforeCompaction Number of Full GCs before compaction 0
-XX:ConcGCThreads Concurrent GC thread count (ParallelGCThreads+3)/4
-XX:+CMSParallelRemarkEnabled Parallel remark Enabled
-XX:+CMSParallelSurvivorRemarkEnabled Parallel Survivor remark Enabled

G1 Parameters

Parameter Description Default
-XX:MaxGCPauseMillis Target pause time 200ms
-XX:G1HeapRegionSize Region size Auto-calculated
-XX:InitiatingHeapOccupancyPercent Heap occupancy to trigger concurrent marking 45%
-XX:G1MixedGCCountTarget Mixed GC count target 8
-XX:G1MixedGCLiveThresholdPercent Region liveness threshold for reclaim 85%
-XX:G1ReservePercent Reserved space to prevent promotion failure 10%
-XX:ParallelGCThreads STW phase parallel thread count CPU cores
-XX:ConcGCThreads Concurrent phase thread count max(1, ParallelGCThreads/4)

ZGC Parameters

Parameter Description Default
-XX:ZCollectionInterval GC interval (0 = adaptive) 0
-XX:ZAllocationSpikeTolerance Allocation spike tolerance 1
-XX:ZFragmentationLimit Fragmentation limit 5%
-XX:+ZGenerational Generational mode (JDK 21+) Disabled

JIT Compilation Parameters

Parameter Description Default
-XX:+TieredCompilation Tiered compilation Enabled
-XX:CompileThreshold Method invocation count for JIT 10000
-XX:CompileThresholdScaling Threshold scaling factor 1.0
-XX:+PrintCompilation Print JIT compilation log Disabled
-XX:ReservedCodeCacheSize Code cache size 240MB
-XX:+Inline Method inlining Enabled
-XX:MaxInlineSize Max bytecode size for inlined methods 35
-XX:FreqInlineSize Inline size limit for frequently called methods 325 
# JIT debugging
-XX:+PrintCompilation          # View which methods are JIT-compiled
-XX:+UnlockDiagnosticVMOptions # Unlock diagnostic options
-XX:+LogCompilation            # Detailed compilation log (XML format)

Thread Parameters

Parameter Description Default
-Xss Thread stack size Platform-dependent
-XX:ThreadStackSize Thread stack size (same as -Xss) Platform-dependent
-XX:+UseThreadPriorities Use thread priorities Enabled 
# Thread stack calculation for high-concurrency applications
# Available memory = Physical memory - Heap - Metaspace - Code cache - OS reserved
# Max threads ≈ Available memory / Xss
# Example: (8G - 4G - 512M - 240M - 500M) / 512K ≈ 5400 threads

Diagnostic Parameters

Parameter Description
-XX:+HeapDumpOnOutOfMemoryError Auto heap dump on OOM
-XX:HeapDumpPath=/path/to/dump Dump file path
-XX:ErrorFile=/path/to/hs_err.log Fatal error log path
-XX:+PrintGCDetails Print GC details (JDK 8)
-XX:+PrintGCDateStamps Print GC timestamps (JDK 8)
-Xlog:gc*:file=gc.log GC log (JDK 9+)
-XX:+PrintSafepointStatistics Print safepoint statistics
-XX:+PrintTLAB Print TLAB information
-XX:+TraceClassLoading Trace class loading
-XX:+TraceClassUnloading Trace class unloading

Parameter Quick Reference by Scenario

By Use Case

Web Service (Spring Boot)

java -Xms2g -Xmx2g \
     -XX:+UseG1GC \
     -XX:MaxGCPauseMillis=200 \
     -XX:MetaspaceSize=256m -XX:MaxMetaspaceSize=256m \
     -XX:+HeapDumpOnOutOfMemoryError \
     -XX:HeapDumpPath=/var/log/heapdump.hprof \
     -Xlog:gc*:file=/var/log/gc.log:time,uptime:filecount=5,filesize=20m \
     -jar app.jar

Big Data (Spark/Flink)

java -Xms8g -Xmx8g \
     -XX:+UseG1GC \
     -XX:MaxGCPauseMillis=100 \
     -XX:G1HeapRegionSize=16m \
     -XX:InitiatingHeapOccupancyPercent=35 \
     -XX:ParallelGCThreads=8 \
     -XX:ConcGCThreads=2 \
     -XX:+HeapDumpOnOutOfMemoryError \
     -jar app.jar

Low-Latency (Trading Systems)

java -Xms4g -Xmx4g \
     -XX:+UseZGC \
     -XX:ZAllocationSpikeTolerance=2 \
     -XX:ConcGCThreads=2 \
     -XX:+UnlockDiagnosticVMOptions \
     -XX:+ZStatisticsForceTrace \
     -XX:+HeapDumpOnOutOfMemoryError \
     -jar app.jar

Microservice (Docker Container)

java -Xms512m -Xmx512m \
     -XX:+UseG1GC \
     -XX:MaxGCPauseMillis=100 \
     -XX:MetaspaceSize=128m -XX:MaxMetaspaceSize=128m \
     -XX:+UseContainerSupport \
     -XX:MaxRAMPercentage=75.0 \
     -jar app.jar

Container-Aware Parameters

Parameter Description Default
-XX:+UseContainerSupport Enable container awareness Enabled (JDK 10+)
-XX:MaxRAMPercentage Max heap as percentage of container memory 25%
-XX:InitialRAMPercentage Initial heap as percentage of container memory 25%
-XX:MinRAMPercentage Min heap as percentage of container memory 50% 
# Docker container recommendation (4GB container memory)
-XX:+UseContainerSupport
-XX:MaxRAMPercentage=75.0     # 3GB heap
-XX:InitialRAMPercentage=75.0 # Initial heap = max heap

Viewing Runtime Parameters

# View all JVM parameters (including defaults)
java -XX:+PrintFlagsFinal -version | grep -i "g1\|heap\|gc"

# View user-modified parameters only
java -XX:+PrintCommandLineFlags -version

# View at runtime
jcmd <pid> VM.flags
jcmd <pid> VM.command_line
jinfo -flags <pid>

Summary

This chapter provided a systematic guide to JVM tuning parameters: memory parameters control the size of each area, GC parameters select the collector and adjust strategies, JIT parameters optimize compilation behavior, and diagnostic parameters help with troubleshooting. In production, it’s recommended to start with scenario-based presets and then fine-tune based on actual monitoring data. The next chapter ties all the knowledge together through comprehensive tuning case studies.

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