Java virtual machine
Virtual machine
- Memory allocation facilities
- Simplified programming interface
- Compile once, run everywhere
Memory allocation
Garbage collection
The HEAP
GC Roots
- Local variables
- Static variables
- Active threads
- JNI references
- Local scope
- Class scope
- Runtime/Manual
- Manual
Good practices
- Always set -Xms and -Xmx flags
- Avoid global state
- Avoid long-running processes
- Keep it as local as possible
- Mark elements for garbage collection
- Create-new is simpler than re-use
Garbage collection
Cleanup while working
- Create objects
- Use them
- Pause the world and check them
- Remove the orphans
- Resume the world
- Start again
Define your collector
| Collector | Scenario |
|---|---|
| SerialGC | Single processor |
| Serial GC | 100 MB of RAM or less |
| Parallel GC | Performance, response time |
| Parallel GC + MarkSweep GC | Performance, short garbage collection period |
Define your garbage collection period
- New ratio
- Survivor ratio
- Pause between collections
- Disable explicit GC
Dump on error
- Dump heap on memory error
- On critical scenarios turn on GC-log
- Use a specific GC-log file
Must have production flags
| Flag | Flags |
|---|---|
| -server | -XX:+CMSParallelRemarkEnabled |
| -Xms<heap size>[g|m|k] | -XX:ParallelGCThreads=<amount of threads> |
| -Xmx<heap size>[g|m|k] | -XX:MaxGCPauseMillis |
| -XX:PermSize=<perm gen size>[g|m|k] | -XX:MaxPermSize |
| -XX:MaxPermSize=<perm gen size>[g|m|k] | -XX:+CMSClassUnloadingEnabled |
| -Xmn<young size>[g|m|k] | -XX:+CMSPermGenSweepingEnabled |
| -XX:SurvivorRatio=<ratio> | -XX:+DisableExplicitGC |
| -XX:+UseConcMarkSweepGC | -XX:+HeapDumpOnOutOfMemoryError |
| -XX:+ScavengeBeforeFullGC | -XX:HeapDumpPath=<path to dump>`date`.hprof |
Good practices
- Never use defaults for production
- Consider memory and process capacities
- Always dump on error
- Always enable GC logging on error
- Do not manually call GC
Tooling
Memory usage/allocation
- JMAP
- Eclipse MAT
- YourKit Profiler
Monitoring
- JMX + JConsole
- JavaMelody
- Perf4j
Right now the JVM is perhaps the best virtual machine out there, hands down
Languages summit 2013