Why is creating a Thread said to be expensive?

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The Java tutorials say that creating a Thread is expensive. But why exactly is it expensive? What exactly is happening when a Java Thread is created that makes its creation expensive? I'm taking the statement as true, but I'm just interested in mechanics of Thread creation in JVM.

Thread lifecycle overhead. Thread creation and teardown are not free. The actual overhead varies across platforms, but thread creation takes time, introducing latency into request processing, and requires some processing activity by the JVM and OS. If requests are frequent and lightweight, as in most server applications, creating a new thread for each request can consume significant computing resources.

From Java Concurrency in Practice
By Brian Goetz, Tim Peierls, Joshua Bloch, Joseph Bowbeer, David Holmes, Doug Lea
Print ISBN-10: 0-321-34960-1

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There are two kinds of threads:

Proper threads: these are abstractions around the underlying operating system's threading facilities. Thread creation is, therefore, as expensive as the system's -- there's always an overhead.
"Green" threads: created and scheduled by the JVM, these are cheaper, but no proper paralellism occurs. These behave like threads, but are executed within the JVM thread in the OS. They are not often used, to my knowledge.

The biggest factor I can think of in the thread-creation overhead, is the stack-size you have defined for your threads. Thread stack-size can be passed as a parameter when running the VM.

Other than that, thread creation is mostly OS-dependent, and even VM-implementation-dependent.

Now, let me point something out: creating threads is expensive if you're planning on firing 2000 threads per second, every second of your runtime. The JVM is not designed to handle that. If you'll have a couple of stable workers that won't be fired and killed over and over, relax.

answered 2011-03-30T07:17:04.347

Why is creating a Thread said to be expensive?

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