PHP garbage collection mechanism use case analysis

This time I will bring you a case analysis of the use of the PHP garbage collection mechanism. What are the precautions when using the PHP garbage collection mechanism? The following is a practical case, let's take a look.

1. When each variable is defined, it is stored in a container called zval, which contains the type and value of the quantity, as well as a refcount (understood as the number of variables) and is_ref (understood as whether it is a reference variable) has two additional information. When the variable is referenced once, the refcount will be 1. When you unset it, the value will be reduced by 1 until it is 0 and it will be deleted from the memory.

2. When defining a variable, the predetermined value will not be expanded every time, because PHP will pre-occupy a space in the memory, and it will be allocated to you when you declare the variable, but when you exceed this pre-occupied space , then it will increase the space, but the space capacity will not disappear immediately when you delete the variable

3. The reference of the variable will not increase the memory usage alone, it will point to the zval structure , just refcount 1

4. To put it simply, PHP’s variables rely on an internal implementation of the symbol_table symbol table, and the basic implementation of the symbol table is HashTable, which is and# The basic implementation of ##PHP array is consistent. It is really because of the existence of the symbol table that we can use global to mark global variables and use functions such as compact to directly pull variables out of the current symbol table.

Let’s talk about whether unset($a) will release the space immediately. The answer is no. Unset supports deleting the element named a from the symbol table (it just marks The space is available again instead of freeing up space).

Let’s talk about the situation of repeatedly updating $key in the loop. Because the variables with the same name are updated, they are the same element in the symbol table. When updating, the same position will be updated, and the value of the previous element will be updated. It was immediately covered.

Let’s talk about the problem that memory will increase if new variables are declared. The answer is uncertain. This is due to the characteristics of the symbol table implemented based on HashTable. HashTable does not apply for memory for one element when adding an element, but applies for memory for multiple elements at once (only these position marks are unused). When HashTable is filled, When, apply for new memory for multiple elements. That is to say, when we declare or assign a variable, if it is not in the symbol table, PHP will add it to the symbol table. If the symbol table is not full at this time, it will use the applied but unused symbol table. If the symbol table is just full, new memory will be applied for storage, and the new memory is not only as small as the memory required for this variable. I believe you have mastered the method after reading the case in this article. , for more exciting content, please pay attention to other related articles on the php Chinese website!

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