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javamultithreadingprocess
Philipp Spiess
Philipp Spiess

Reputation: 3271

Does a Java Thread have its own process ID?

I want to get the process ID of a Thread to see how much memory it takes.

Upvotes: 9

Views: 11120

Answers (4)

Joachim Sauer
Joachim Sauer

Reputation: 308061

It depends a lot on the OS and how it manages threads. Theoretically it also depends on how the JVM implements threads, but all modern JVMs implement them as native threads.

On Linux each thread will used to get its own process ID, but most tools hide all but one thread per process (i.e. you don't usually see them unless you explicitly ask for them, ps uses the -m flag for example). This is caused by the fact that the Linux kernel doesn't really make much of a difference between threads and tasks.

Edit: as I just learned this is no longer necessarily the case: you can create a thread with the exact same PID as the parent, in which case the threads will be distinguished by different thread IDs.

However since a thread shares its memory with all other threads in the same process, this doesn't help you find out "how much memory a thread takes", since all threads in a process will use the exact same amount (and they all use the same, so the real used memory is shown_memory_use and not shown_memory_user * number_of_threads).

Upvotes: 14

Peter Lawrey
Peter Lawrey

Reputation: 533530

A feature of thread is that is shares the heap with all other threads. This means that any one thread can potentially use almost all the memory of the process. The only thing which a thread doesn't have access to is the stack or local variables of another thread.

As such it is not useful to try to determine how much memory an individual thread uses. Instead it can be useful to determine how much memory a data structure uses. (Although this can have similar difficulties)

It is worth noting that main memory is relatively cheap. Your situation may be different but a typical new server with 24 GB can cost as little as £1K. You can buy a 96 GB PC for around £2K. Sometimes it is not worth worrying about how much memory you are using until you know it is a problem.

Upvotes: 2

Remon van Vliet
Remon van Vliet

Reputation: 18605

Threads do not have PIDs, processes do. As such what you're asking is not possible. There is also no reliable way to retrieve your PID from within a Java process (although the first part of the value returned by ManagementFactory.getRuntimeMXBean().getName() usually is the PID).

Upvotes: 4

helpermethod
helpermethod

Reputation: 62185

As the name implies, PID means process ID. Each process can spawn multiple threads, which all share the same PID. Are you sure you don't mean Thread ID?

Upvotes: 3

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