Difference between javacore,thread dump and heap dump in Websphere

Question

Can someone tell me the exact difference between javacore,thread dump and heap dump.Under which situation each of these are used??

Answer 1

Heap dumps anytime you wish to see what is being held in memory Out-of-memory errors Heap dumps - picture of in memory objects - used for memory analysis Java cores - also known as thread dumps or java dumps, used for viewing the thread activity inside the JVM at a given time. IBM javacores should a lot of additional information besides just the threads and stacks -- used to determine hangs, deadlocks, and reasons for performance degredation System cores

Answer 2

A Thread dump is a dump of all threads's stack traces, i.e. as if each Thread suddenly threw an Exception and printStackTrace'ed that. This is so that you can see what each thread is doing at some specific point, and is for example very good to catch deadlocks.

A heap dump is a "binary dump" of the full memory the JVM is using, and is for example useful if you need to know why you are running out of memory - in the heap dump you could for example see that you have one billion User objects, even though you should only have a thousand, which points to a memory retention problem.

Answer 3

A thread dump is a dump of the stacks of all live threads. Thus useful for analysing what an app is up to at some point in time, and if done at intervals handy in diagnosing some kinds of 'execution' problems (e.g. thread deadlock).

A heap dump is a dump of the state of the Java heap memory. Thus useful for analysing what use of memory an app is making at some point in time so handy in diagnosing some memory issues, and if done at intervals handy in diagnosing memory leaks.

This is what they are in 'raw' terms, and could be provided in many ways. In general used to describe dumped files from JVMs and app servers, and in this form they are a low level tool. Useful if for some reason you can't get anything else, but you will find life easier using decent profiling tool to get similar but easier to dissect info.

With respect to WebSphere a javacore file is a thread dump, albeit with a lot of other info such as locks and loaded classes and some limited memory usage info, and a PHD file is a heap dump.

If you want to read a javacore file you can do so by hand, but there is an IBM tool (BM Thread and Monitor Dump Analyzer) which makes it simpler. If you want to read a heap dump file you need one of many IBM tools: MDD4J or Heap Analyzer.