Memory management and garbage collection technology in PHP

PHP, as a widely used scripting language, has unique memory management and garbage collection technology in order to ensure efficient execution at runtime. This article will briefly introduce the principles and implementation methods of PHP memory management and garbage collection.

1. Principle of PHP memory management

PHP’s memory management is implemented by reference counting. This method is one of the more common memory management methods in modern languages. . When a variable is used, PHP allocates a memory for it and sets the reference count of this memory to 1. Whenever the value of one variable is copied to another variable, PHP increments the reference count of that memory block. When a variable is no longer used, PHP decrements its reference count by 1. When the reference count reaches 0, the memory block is released.

This reference counting method is suitable for memory management in most cases, but circular references may also occur. Suppose we have two variables a and b, which refer to each other, then their reference count is never 0, and the memory block can never be released. To solve this problem, PHP introduced a garbage collection mechanism.

2. PHP's garbage collection mechanism

In PHP's garbage collection mechanism, memory blocks that are no longer used are marked and cleared by traversing the variables in the memory. There are two specific implementation methods:

1. Mark and Sweep algorithm

The mark and sweep algorithm is the most common method in garbage collection. Its basic idea is to run Record the usage of each memory block at all times. When a memory block is no longer referenced, it is marked as unused and recycled.

The algorithm requires two phases, marking and clearing. In the marking phase, the GC will traverse the entire memory space and mark all active objects, while in the clearing phase, the GC will clear all objects marked as inactive.

The advantage is that it can recycle the object memory of circular references, but the disadvantage is that it will cause memory fragmentation and affect the running efficiency of the program.

2. Ref-count and Mark and Sweep algorithm

The reference count plus mark and sweep algorithm is the official implementation of PHP, which combines two The characteristics of this algorithm avoid their shortcomings and achieve efficient memory management.

In this algorithm, PHP still uses reference counting to manage the increase and decrease of memory. When the reference count of a memory block reaches 0, the memory block will be marked as inactive, and then the mark and clear operations will be performed.

This method combines the advantages of the two algorithms. Memory block recycling is more efficient and does not produce a large number of memory fragments.

3. Best practices for PHP memory management

The implementation of PHP memory management determines some issues we should pay attention to when writing PHP programs. In practice, the following optimizations can be taken:

1. The allocation of small blocks of memory should be avoided as much as possible

Because PHP uses a heap memory management method, if Frequent application and release of memory will cause memory fragmentation and increase the burden of GC. Therefore, frequent application and release of small blocks of memory should be avoided during program design.

2. If you need to apply for and release memory frequently, you can use object pool

Object pool is a commonly used memory management technology. When you need to frequently allocate and release memory, , you can put objects into the object pool and reuse these objects. This technology can improve the efficiency of memory allocation and recycling and reduce the burden of GC.

3. The problem of circular references needs to be paid attention to

In actual development, circular reference scenarios are very common, such as foreign key relationships in the database. If a circular reference occurs, you can use the weakref function to solve it. weakref creates a weak reference object that does not affect the reference count of the target object.

Summary:

PHP’s memory management and garbage collection technology is one of the important ways to achieve efficient code. Using these technologies well can reduce the burden of GC and improve program performance. During development, we need to pay attention to avoid circular references as much as possible, pay attention to the life cycle of memory objects, avoid frequent allocation and release of memory, and reduce the pressure on GC.

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