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.

1. Java Performance Debugging

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

10. Java Heap Structure

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