Java Troubleshooting and Performance
Margus Pala
Java Fundamentals 08.12.2014
Agenda
● Debugger
● Thread dumps
● Memory dumps
● Crash dumps
● Tools/profilers
Rules of (performance) optimization
● 1. Don't optimize
● 2. Don't optimize yet (For experts only)
--Unknown
● Hardware is Cheap, Programmers are Expensive
--Jeff Atwood
Types of performance bottlenecks
● Low memory – GC performance
– Lot of GC flags to tune
● Thread contention
● Ineffective algorithm
– Unoptimized big collection operation
– Too many objects created and dropped
Low tech tips
● Print statuses
– Activity and variable values
– Visited methods
– To System.out, System.err or log
● Throw and print exceptions
● Measure time
– System.currentTimeMillis()
● Break it down to single statement
Rules of (performance) optimization
● 1. Don't optimize
● 2. Don't optimize yet (For experts only)
--Unknown
● Hardware is Cheap, Programmers are Expensive
--Jeff Atwood
Common GC tuning flags and strategies
● Initial vs max sizes of memory areas
– -Xms512m –Xmx512m (avoids JVM own optimizations)
● Relative sizes of old and new areas (short or long living obj)
– -XX:NewRatio=8
– -XX:NewSize=512m
– -XX:MaxNewSize=512m
● Collector type
– -XX:+UseParallelOldGC (high throughput)
– -XX:+UseConcMarkSweepGC (low latency)
– -XX:+UseG1GC (predictable low latency)
Debugging Basics
● Set breakpoints
● Run in debug mode
● Controlling execution
– Step into
– Step over
– Step out
– Resume
– Suspend
– Stop
Breakpoints view
● Allows to control breakpoints in one place
– Enable/Disable
– Delete
– Delete all
– Skip all
● Setup conditional breakpoints
Variables view
● Shows list of variables and their values
● Can edit variable values
● Use “View Menu” up right to config
– Show constants
– Show static values
● “view menu” → layout → columns
– Enables to choose different data to be shown
● Detail formatter
– Choose class method to return more clear representation
Advanced debugging
● Conditional breakpoint
– Based on hit count
– Based on coded condition
● Suspend when true
● Suspend when condition value changes
● Watchpoint
– Read or write the field
– On Instance or static non-final variable
Other breakpoints
● Exception breakpoint
– Added from breakpoints view
– Suspend on caught exceptions
– Suspend on uncaught exceptions
● Method breakpoint
– Added from left margin of method header
– Suspend on method entry
– Suspend on method exit
● Load class breakpoints
– Added from outline, rightclick on class
Step Filter
● Allows skipping framework classes while stepping into
● Window → Preferences → Java → Debug → Step filtering
Remote debugging
● Allows to debug JVM on another machine
● Needs JVM arguments:
-Xdebug -Xnoagent -Djava.compiler=NONE
-Xrunjdwp:transport=dt_socket,server=y,suspe nd=y,address=5005
● Needs “Remote Java Application” type debug configuration
Drop to frame
● Allows to go back in execution to previous method in stack.
● Variable modifications will not be reversed
Thread dump
● All threads stacks
● Generate with:
– jstack pid
– Kill -3 pid
– Win: ctrl + pause/break
– Tools (visualvm, ..)
Problems analyzable by thread dumps
● Deadlock
● Thread contention
– Low performance
● CPU usage without profilers.
– Take multiple thread dumps and compare
Thread statuses
● NEW: The thread is created but has not been processed yet.
● RUNNABLE: The thread is occupying the CPU and processing a task.
● BLOCKED: The thread is waiting for a different thread to release its lock in order to get the monitor lock.
● WAITING: The thread is waiting by using a wait, join or park method.
● TIMED_WAITING: The thread is waiting by using a sleep, wait, join or park method. There is max time.
Thread types
● Non-Daemon
– Main thread
● Daemon
– Will stop running If all non-daemon threads have finished running
– Usually several created if only one explicit thread
Memory dump (Heap dump)
● Heap dump in hprof format
– JVM details
– System properties
– Thread dump ( JVM 1.6+ )
– Snapshot of all objects.
● Generate with
– jmap -dump:[live,]file=<filename> pid
– -XX:+HeapDumpOnOutOfMemoryError
Analyzing memory dumps
● Find memory leaks
● Detect large objects
– High memory usage
● Save the memory state for later debugging
Java crash files
● hs_err_pid*.log
– Fatal error log
● Core dump
– Heap dump and more
Fatal Error Log
● Is created when JVM crashes
● Shows low level info to understand the root cause of crash
● Saved as hs_err_pid<nr>.log or XX:ErrorFile=
– In working directory or /tmp
Fatal error log contents
● The operating exception or signal that provoked the fatal error
● Version and configuration information
● Details on the thread that provoked the fatal error and thread's stack trace
● The list of running threads and their state
● Summary information about the heap
● The list of native libraries loaded
● Command line arguments
● Environment variables
● Details about the operating system and CPU
MBeans aka JMX API
● Give additional information about JVM
– Memory and GC
● Execute GC
– CPU
– System
– Threads
● Many tools work on top of it
● Allows to run operations
Tools
● jconsole
– Monitoring tool
– MBeans
● jvisualvm
– “All-in-one java troubleshooting tool”
– Lightweight profiler
– Plugin system (VisualGC)
● Java Mission Control
– Advanced monitoring
– Flight recorder
Homework 14
● https://github.com/zeroturnaround/jf-hw-performance
● This project has an implemention for the N-puzzle
( http://en.wikipedia.org/wiki/Fifteen_puzzle ). The board is a little bit bigger though, 5x5. The project uses the class org.zt.jf.perf.Main for benchmarking the implementation. It uses the A* algorithm by default but is bundled with depth first and breadth first approaches also.
● Your task is to
– Probably you want some custom flags here.
– Describe in a sentence or two why the program is as slow as it is
– Make whatever source code changes you see fit so that the program would run
faster. Of course it needs to produce the same results and solve the same problem :)
– Please provide the code changes and also measurements as how much were you able to improve the speed
References
● Eclipse debugging tutorial http://goo.gl/cxIGmf
● Fatal error log http://goo.gl/xepBPL
