What are the PHP garbage collection projects?

php garbage collection projects include: 1. Reference counting, by counting the references to variables, when the count is 0, the variable will be released; 2. Mark clearing, by traversing all reachable objects, the mark is Reference the object, and then clear the unmarked objects to ensure the release of memory that is no longer used; 3. Generational recycling, divide the objects into different generations, each generation has its own garbage collection strategy, when the first generation is full, garbage will be triggered Recycling; 4. Memory pool, which divides memory into fixed-size blocks. Each block has a mark indicating whether it is used. When the object is no longer used, it will be marked as free.

The operating environment of this article: Windows 10 system, PHP8.1.3 version, Dell G3 computer.

PHP is a scripting language with an automatic garbage collection mechanism that can help developers automatically release memory that is no longer used. The main types of garbage collection in PHP are as follows:

1. Reference Counting Garbage Collection: The earliest garbage collection mechanism adopted by PHP is reference counting. When a variable is referenced, the reference count is incremented by 1; when a variable is no longer referenced, the reference count is decremented by 1. When the reference count reaches 0, it means that the variable is no longer used and the memory can be released. This method is simple and efficient, but it has the problem of circular reference, that is, two or more objects refer to each other, causing the reference count to never be 0 and the memory cannot be released.

2. Mark and Sweep Garbage Collection: In order to solve the problem of circular references, PHP introduces a mark and sweep garbage collection mechanism. This mechanism ensures the release of memory that is no longer used by traversing all reachable objects, marking referenced objects, and then clearing unmarked objects. However, it requires traversing the entire object graph, and the processing efficiency of large object graphs is low.

3. Generational Garbage Collection: In order to improve the efficiency of garbage collection, PHP has incorporated a generational collection mechanism. This mechanism divides objects into different generations, and each generation has its own garbage collection strategy. Generally speaking, newly created objects will be put into the first generation. When the first generation is full, garbage collection will be triggered to recycle objects that are no longer used. If an object survives multiple garbage collections, it is promoted through generations until it reaches the oldest generation. This can reduce the frequency of garbage collection and improve recycling efficiency.

4. Memory Pool Garbage Collection: In order to avoid the generation of memory fragments, PHP introduces a memory pool garbage collection mechanism. This mechanism divides memory into fixed-size blocks, and each block has a flag indicating whether it is being used. When an object is created, a block is allocated from the memory pool, and when the object is no longer used, the block is marked free. This can reduce the generation of memory fragmentation and improve memory utilization.

Summary

PHP’s garbage collection projects mainly include reference counting, mark removal, generational recycling and memory pools. Each garbage collection mechanism has its advantages and disadvantages, and developers can choose the appropriate garbage collection mechanism based on specific needs.

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