Cache data alignment in Java caching technology

Cache data alignment in Java caching technology

In Java caching technology, data alignment is a very key concept for caching data. Data alignment makes full use of the CPU's cache mechanism to improve data access efficiency. This article will briefly introduce cache data alignment in Java caching technology.

1. What is data alignment

Data alignment means that when storing data in memory, the data is stored continuously starting from the starting address. The CPU cache generally stores data in groups of 64 bytes, so that data can be stored continuously starting from the starting address, so that you can make full use of this caching mechanism in groups of 64 bytes. In addition, aligned data can also reduce the time delay of data access and improve the running efficiency of the program.

For example, we consider a structure containing two int type variables. On a 32-bit system, the size of each int type variable is 4 bytes, so the total space occupied by the structure is 8 words. Festival. If this structure were stored at address 0 in memory, since 8 is not a multiple of 64, it would span two cache lines, resulting in cache misses and performance degradation. And if this structure is stored at an address that is a multiple of 8, data alignment can be ensured, and the CPU's cache mechanism can be fully utilized to improve performance.

2. Data alignment in Java

In Java, cache data alignment is generally done by the JVM, and developers only need to comply with some specifications. Specifically, the following points need attention:

(1) Alignment of basic type variables

The size of Java's basic type variables in memory is fixed, such as the size of byte type variables is 1 byte, and the size of int type variable is 4 bytes. In an array, consecutive elements will be allocated in consecutive memory addresses. Therefore, if the starting address of the array is a multiple of 4, then the starting address of each element of the array is also a multiple of 4, which can be achieved Data alignment.

(2) Alignment of object header

In Java, each object has an object header, which is used to store the meta-information of the object, such as the object's class information, lock information, etc. . The size of the object header depends on the specific JVM implementation. Generally speaking, the object header size is 8 bytes or 12 bytes, and must be a multiple of 8. When the JVM allocates an object, it finds an aligned location in available memory to place the object, ensuring that the object is head-aligned.

(3) Alignment of member variables of objects

In Java, the size of member variables of an object is variable, depending on the specific type and JVM implementation. When storing objects, the JVM generally uses a technology called "memory packing" to use data alignment to reduce memory waste. When an object's member variables are not of a basic type, the JVM will choose the alignment method according to a certain strategy to ensure data alignment.

In short, when using Java caching technology, we need to pay attention to the concept of data alignment and make full use of the CPU's caching mechanism to improve the running efficiency of the program. At the same time, pay attention to code specifications and try to comply with JVM specifications to achieve data alignment.

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