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JVM Profiling: Thread Dumps

Thread dumps are possilby the most useful tool in diagnosing concurrancy related issues. You can get a thread dump at any time by sending a break signal to the JVM. In Windows it's CTRL-Break and in Unix/Linux it's kill -3 <pid>

The possible issues to look for are:

  • Deadlocks: You 'll find something similar to "Found one Java-level deadlock:" in the tread dump. Have a look here for an explanation of deadlocks.
  • Blocked: If there a number of threads blocked then look in the thread dump if they are blocked on the same monitor. This will indicate a heavily contented resource. You can see which thread has locked on the monitor by searching for "locked <monitor_id>". You may want to review the design of this code (if you can change it) to ensure that you're not over-zealous with locking or that the locking thread is not blocking on another monitor. Have a look at Software Transactional Memory for a possible alternative to locking.

You may also use "Thread Telemetry" view avialable in most JVM profiling tools (e.g. JProbe, JProfiler, YourKit etc.). This gives you an historical view of thread states in the JVM. Very useful for monitoring JVM performance over a period of time and spotting live-lock situations.

Sobre el autor

Mash is a pragmatic software craftsman always looking to improve his software creation skills and helping others do the same. He firmly believes that a well-rounded software craftsman must have a keen interest in all aspects of software creation, including; process, people, technology, user experience, development, operation, maintenance, and social impact. He relishes the daily challenges that Codurance brings to him–stretching his existing knowledge and expertise allowing him to constantly grow as a professional.

Mash is an advisor and a leader. During his diverse career, he has succeeded in invigorating large ailing software projects as well as creating highly effective software teams and departments. His broad and deep technical knowledge, organisational skills, craft focus, and empathy to people involved have been integral to his success. He has worked in many roles for charities, investment banks, consultancies, government, media and cloud providers. He prides himself at being a hands-on software developer and believes that software development skills are very hard to learn and the best way to maintain them is to apply them.

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