Why is Creating a Thread in Java Considered Expensive?

Why is Thread Creation Deemed Costly?

While the Java tutorials state that creating a Thread incurs a significant cost, the actual reason behind this expense has remained unclear. This article delves into the mechanics of Thread creation in the JVM to uncover the factors that contribute to its costly nature.

Platform-Dependent Overhead

Thread creation entails overhead that varies across platforms. Establishing a native thread within the host OS, allocating a large memory block for the thread's stack, and initializing JVM and OS data structures all add to this overhead.

Resource Consumption

Once created, a native thread consumes considerable resources throughout its lifetime. These include the allocated thread stack, objects accessible from the stack, JVM thread descriptors, and OS native thread descriptors.

Benchmark Evidence

A benchmarking exercise revealed that thread creation occurred at a rate of approximately 4000 per second on a dual-processor Xeon system running Java 1.4.1 in 2002. While modern platforms offer improved performance, thread creation remains relatively expensive.

Virtual Threads and Project Loom

Java platforms have traditionally employed native threads for performance reasons. However, the OpenJDK Loom project introduces virtual threads as a potential solution. These threads are hybrids that combine native threads and green threads, aiming to reduce the overhead associated with native thread creation while maintaining concurrency.

Stack Allocation in OpenJDK

OpenJDK 6 on Linux employs mmap to allocate stack memory for a native thread through the pthread_create call. This memory is automatically initialized to zero, aligning with the assumptions made in the JVM specification regarding the zeroing of new Java thread stacks.

In summary, thread creation is considered expensive due to the substantial overhead involved in establishing a new native thread, allocating a large memory block, and consuming various resources across its lifetime. Additionally, platform-specific factors influence the precise cost of thread creation.

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