Why is Creating a Java Thread so Expensive?

Why Creating a Thread is Expensive

Java's documentation states that thread creation is an expensive operation. But what exactly makes it costly?

Thread Lifecycle Overhead

Creating and terminating threads incurs significant overhead. The JVM and operating system engage in various operations, such as:

• Allocating and initializing a large memory block for the thread stack
• Making system calls to register the native thread with the host OS
• Creating and initializing descriptors for the thread within JVM data structures

Resource Consumption

Threads consume resources throughout their lifespan, including:

• Thread stack memory
• Objects reachable from the stack
• JVM and OS thread descriptors

Platform Variability

The specific costs vary across platforms. However, thread creation is generally not a cheap operation on any Java platform.

Benchmarking Data

Benchmarking indicates that creating a thread on a Sun Java 1.4.1 platform on a 2002 Xeon processor resulted in a creation rate of approximately 4000 threads per second. More modern platforms offer faster creation times, but even so, thread creation remains significantly more expensive than reusing threads from a thread pool.

Green Threads vs. Native Threads

Green threads, which are implemented in software and do not rely on native threads, may be cheaper to create. However, they come with other performance trade-offs.

Project Loom

The OpenJDK Loom project aims to introduce lightweight virtual threads as an alternative to standard Java threads. Virtual threads combine aspects of native and green threads, offering potential cost reductions.

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