JVM local memory

Jul 27, 202361

AI Translation

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AI-generated summary

The concept of local memory is that it consists of direct memory and metaspace. Direct memory is a memory area outside of the Java heap that is directly allocated from the system. It is used for high-performance reading and writing through DirectByteBuffer in the heap. The reason for using direct memory is that it is a physical memory specifically used for communication between the JVM and IO devices, avoiding data corruption during garbage collection. Metaspace, introduced in JDK 1.8, stores loaded class information, constants, static variables, constant pool, and compiled code. Each thread in the Java Memory Model (JMM) has its own working memory for storing shared variables in the JVM. Working memory is an abstract concept that includes CPU registers and caches.

Concept#

Local memory = Direct memory + Metaspace

Direct memory#

Concept: Direct memory is not part of the virtual machine's runtime data area. It is a memory area that is directly allocated from the system outside of the Java heap. Direct memory uses NIO and operates on native memory through DirectByteBuffer stored in the heap, resulting in high read and write performance.
Reason: Direct memory is a physical memory specifically used for JVM and IO device interaction. Since JVM operates based on automatic garbage collection, data in memory is constantly moved during GC. If you call a system API to write the memory at a certain location to disk, and at the same time GC moves that data, the operating system may write incorrect data after GC finishes.

Metaspace#

Concept: The concept of Metaspace appeared in JDK 1.8, previously known as the method area/permanent generation. It stores loaded class information, constants, static variables, constant pool, and JIT-compiled code.

Working memory and local memory#

Each thread in JMM has its own working memory to store shared variables in JVM memory. Each thread can only operate on variables in its own working memory. Working memory is an abstract concept, not actual memory, but rather CPU registers and caches.