JVM Tuning for Cassandra

Optimize JVM settings for optimal Cassandra performance.

Heap Configuration

Sizing Guidelines

System RAM	Heap Size	Notes
8GB	4GB	Development only
32GB	8GB	Small production
64GB	16GB	Standard production
128GB+	31GB	Maximum recommended

Key Rules:

• Never exceed 31GB (compressed oops limit)
• Leave 50%+ RAM for OS page cache
• Set -Xms equal to -Xmx

Configuration

# jvm11-server.options or jvm17-server.options

# Heap size (adjust for your system)
-Xms16G
-Xmx16G

Garbage Collection

G1GC (Default, Recommended)

# G1GC Configuration
-XX:+UseG1GC
-XX:MaxGCPauseMillis=500
-XX:G1RSetUpdatingPauseTimePercent=5
-XX:InitiatingHeapOccupancyPercent=70
-XX:ParallelGCThreads=16
-XX:ConcGCThreads=4
-XX:+ParallelRefProcEnabled
-XX:MaxTenuringThreshold=1
-XX:G1HeapWastePercent=10

ZGC (Low Latency)

# ZGC for ultra-low latency
-XX:+UseZGC

# ZGenerational is available in JDK 21+ only
# -XX:+ZGenerational

-XX:SoftMaxHeapSize=28G
-XX:ZCollectionInterval=0

ZGC Version Requirements

• JDK 11-16: ZGC is experimental (-XX:+UnlockExperimentalVMOptions required)
• JDK 17-20: ZGC is production-ready (non-generational)
• JDK 21+: ZGenerational mode available via -XX:+ZGenerational

Shenandoah (JDK 11+, Alternative)

# Shenandoah GC
-XX:+UseShenandoahGC
-XX:ShenandoahGCHeuristics=compact

GC Logging

JDK 11+ Format

# Comprehensive GC logging
-Xlog:gc*:file=/var/log/cassandra/gc.log:time,uptime,level,tags:filecount=10,filesize=10M
-Xlog:safepoint:file=/var/log/cassandra/safepoint.log:time,uptime:filecount=5,filesize=5M

Memory Settings

Metaspace

-XX:MetaspaceSize=128M
-XX:MaxMetaspaceSize=256M

Direct Memory (Off-Heap)

# For memtable_offheap and file cache
-XX:MaxDirectMemorySize=4G

Thread Stack

# Reduce stack size for many threads
-Xss256k

Performance Optimizations

String Optimization

# String deduplication (G1GC only)
-XX:+UseStringDeduplication
-XX:StringDeduplicationAgeThreshold=1

Compilation

# Tiered compilation (default)
-XX:+TieredCompilation

# Code cache
-XX:ReservedCodeCacheSize=256M
-XX:+UseCodeCacheFlushing

Disable Explicit GC

# Prevent application-triggered GC
-XX:+DisableExplicitGC

Debugging and Monitoring

Heap Dumps

# Automatic heap dump on OOM
-XX:+HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=/var/lib/cassandra/

# Exit on OOM (let orchestrator restart)
-XX:+ExitOnOutOfMemoryError

JMX (if external)

-Djava.rmi.server.hostname=<node_ip>
-Dcom.sun.management.jmxremote.port=7199
-Dcom.sun.management.jmxremote.ssl=false
-Dcom.sun.management.jmxremote.authenticate=false

Complete Configuration Example

/etc/cassandra/jvm11-server.options

#####################
# HEAP CONFIGURATION
#####################
-Xms16G
-Xmx16G

#####################
# GC CONFIGURATION
#####################
-XX:+UseG1GC
-XX:MaxGCPauseMillis=500
-XX:InitiatingHeapOccupancyPercent=70
-XX:ParallelGCThreads=16
-XX:ConcGCThreads=4
-XX:+ParallelRefProcEnabled
-XX:MaxTenuringThreshold=1
-XX:G1HeapWastePercent=10
-XX:+UseStringDeduplication

#####################
# GC LOGGING
#####################
-Xlog:gc*:file=/var/log/cassandra/gc.log:time,uptime:filecount=10,filesize=10M

#####################
# MEMORY
#####################
-XX:MetaspaceSize=128M
-XX:MaxMetaspaceSize=256M
-XX:MaxDirectMemorySize=4G
-Xss256k

#####################
# ERROR HANDLING
#####################
-XX:+HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=/var/lib/cassandra/
-XX:+ExitOnOutOfMemoryError

#####################
# PERFORMANCE
#####################
-XX:+DisableExplicitGC
-XX:+AlwaysPreTouch
-XX:+UseLargePages

Tuning by Workload

Workload	Heap	GC	Key Settings
Write-heavy	16-24GB	G1GC	Higher IHOP (75%)
Read-heavy	16-31GB	G1GC	Lower pause target
Low latency	16GB	ZGC	-XX:+UseZGC
Mixed	16GB	G1GC	Default settings

Monitoring GC

# Check GC activity
nodetool gcstats

# Watch GC logs
tail -f /var/log/cassandra/gc.log

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Next Steps

• OS Tuning - Operating system optimization
• Hardware - Hardware recommendations
• JVM Options - Complete options reference