3. Problems with I/O
Threads maybe blocked or perform slowly during access to I/O resources:
File system
Network
Database
Single or multiple thread dumps may help understading the problem
4. Problems with
algorithms
Algorithm complexity is not suited well for the data set size
Algorithm is overusing expensive resources or memory
Algorithm is overusing locks, while it can be implemented in a lock-free manner
Single or multiple thread dumps may help noticing the problem
5. Problems with
locks and
syncronization
No locking (not thread-safe implementations)
Deadlocks (overly syncronised)
Lock contention (overly syncronised)
Lock overhead (too many locks)
Single or multiple thread dumps may explain the situation
6. Example:
HashMap gives
100% CPU load
Usage of not syncronised HashMap in a multi-threaded application
In certain situations it goes into infinite loop
This causes 100% CPU load
The probabilty of entering infinite loop depends on the size of the HashMap
Single thread dump reveals the problem
7. From Wikipedia:
Deadlocks
“A deadlock is a situation in which two or more competing actions are each waiting
for the other to finish, and thus neither ever does.
8. Deadlocks
In real life, deadlocks are pretty rare
CPU usage is usally close to 0
Single thread dump can reveal a deadlock situation
9. Lock contention
From Wikipedia:
“This occurs whenever one process or thread attempts to acquire a lock held by
another process or thread. The more granular the available locks, the less likely one
process/thread will request a lock held by the other. (For example, locking a row
rather than the entire table, or locking a cell rather than the entire row.)
10. Lock contention
The most common problem with multi-threaded applications related to lock
handling
Sometimes easy, sometimes very hard to detect
You can only see one layer at a time
Multiple thread dumps may help to detect lock contention situations
11. Sporadic
application
behaviour
May be related to not thread safe code
May be related to load patterns
May be related to specific data
Threads dumps may or may not help
13. Formats
Formats differ for various JVM and application server implementations
There is no tool that supports them all
Though format is usually very readble and follows same principles
16. Thread states II
“
WAITING - A thread that is waiting indefinitely for another thread to perform a
particular action is in this state.
TIMED_WAITING - A thread that is waiting for another thread to perform an
action for up to a specified waiting time is in this state.
TERMINATED - A thread that has exited is in this state.
25. JVM Internals
Training
Aestas/IT organizes "JVM Internals" training on March 25/26, 2013.
This 2–day course takes the developer on an in-depth tour of the Java Virtual Machine.
Intended for experienced JVM-based programmers, who want to get deeper with the
platform, this course will introduce the major subsystems of the JVM and practical ways
to apply this knowledge in real applications.
26. Topics I
History of Java and JVM
Specifications
Bytecode
Base components
Detailed architecture
JIT compilation
Garbage collection
Threads
Locks
Classloading
