Why is Creating a Thread in Java So Expensive?

Why is Thread Creation Expensive in Java?

The Java documentation suggests that creating a thread is an expensive operation. This article explores the reasons behind this claim, delving into the mechanics of thread creation in the Java Virtual Machine (JVM).

At its core, the expensiveness of thread creation lies in the significant overhead it incurs. This overhead includes:

• Memory Allocation: A substantial block of memory must be allocated and initialized for the thread's stack.
• System Calls: System calls are necessary to interact with the host operating system and create the native thread.
• Data Structure Management: Descriptors and data structures need to be created and added to the JVM's internal mechanisms.

Furthermore, a thread represents a persistent commitment of resources during its lifetime. It holds a stack, references objects, and requires maintenance of JVM and operating system descriptors. These resources are forfeited until the thread terminates.

The precise cost of thread creation varies across platforms, but it remains a non-trivial expenditure in all Java environments. To provide a ballpark estimate, an old benchmark on Sun Java 1.4.1 yielded a thread creation rate of approximately 4000 per second. However, this number may fluctuate with Java and operating system improvements or hardware advancements.

In recent times, the OpenJDK Loom project has proposed "virtual threads" as a lightweight alternative to standard Java threads. These virtual threads aim to strike a balance between native threads and green threads, potentially reducing thread creation overhead.

Ultimately, understanding the mechanics behind thread creation is crucial for making informed decisions about thread management. By recognizing the costs associated with creating new threads, developers can optimize their applications and minimize unnecessary overhead.

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