Java performance
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This document discusses how to debug Java performance issues by considering key metrics like throughput, latency, and concurrent users. It outlines potential bottlenecks related to memory, CPU, disk I/O and network I/O. Specific techniques are provided to analyze memory issues using heap dumps and JSTAT, CPU issues using thread dumps, and disk I/O issues using I/O statistics. The document also briefly covers the different garbage collector options in Java.
Java performance
- 2. Few Questions to Consider • What is expected throughput • What is expected latency • Maximum No. of concurrent users/tasks • Expected Throughput/latency at max users • Worst case latency • Tolerable GC latency
- 3. Bottlenecks • Memory • CPU • Disk I/O • Network I/O
- 4. Memory (RAM) • -Xmx too low • Process leaking memory (et tu java!) • Insufficient DirectBuffers • Insufficient PermGen space • Page thrashing
- 5. Memory (Contd.) • Run –XX:+HeapDumpOnOutOfMemoryError. Check .hprof files in working directory or – XX:HeapDumpPath • jmap –histo<pid>
- 6. Memory (contd.) • High si/so coinciding with low ‘free’ memory implies page thrashing
- 7. Other OutOfMemory Errors • Incorrect PermGen size –XXMaxPermSize – java.lang.OutOfMemoryError: PermGen space – jstat -gc<pid> – Can monitory ‘PU’ vs ‘PGCMX’ from jstat - gccapacity • Incorrect Direct Buffer limit - XX:MaxDirectMemorySize – java.lang.OutOfMemoryError: Direct buffer memory
- 8. CPU • Check Garbage collector stats – Jstat –gc<pid> • Investigate using – JMX, TOP – Using TOP get the thread ID – Thread Dump using ‘kill -3’ or ‘jstack’ – Search Hex thread id in thread dump
- 9. Disk I/O • Iostat %iowait<interval><samples> • High %iowait indicates I/O bottleneck • If java process is causing high I/O, check the thread dump
- 11. Garbage Collector Choice • Serial GC (Never use this!) – -XX:+UseSerialGC • Parallel GC (High Throughput) – Use in non- interactive jobs which can tolerate pause of 1- 2 sec – -XX:+UseParallelGC –XX:+UseParallelOldGC • CMS (Low Latency) – Good for webapps and interactive processes – -XX:+UseConcMarkSweepGC