Detailed explanation of PHP copy-on-write
This article mainly shares with you the detailed explanation of PHP copy-on-write. Let's start with an example, hoping to help everyone.
<?php$foo = 1; $bar = $foo;echo $foo + $bar;
The variable $foo is assigned to the variable $bar. These two variables have the same value. There is no need to apply for new memory space. They can share the same memory. In many scenarios, PHP's COW optimizes memory. For example: multiple assignments of variables, passing function parameters, and modifying actual parameters in the function body, etc.
What is "copying"
This is an example taken from Brother Niao's blog. It makes it clearer, so I posted it directly.
<?php $var = "laruence"; $var_dup = $var; $var = 1;?>
Obviously after this code is executed, the value of $var_dup should still be "laruence", so how is this achieved? This is PHP's copy on write mechanism:
Before modifying a variable, PHP will first check the refcount of the variable. If the refcount is greater than 1, PHP will execute a separate routine. For the above code, When executing the third line, PHP finds that the refcount of the zval pointed to by $var is greater than 1, then PHP will copy a new zval, reduce the refcount of the original zval by 1, and modify the symbol_table so that $var and $var_dup Separation. This mechanism is the so-called copy on write.
Copy on Write Application Scenarios
Copy on Write (also abbreviated as COW) has many application scenarios. For example, the optimization of memory usage in process copying in Linux is widely used in various applications. There are similar applications in various programming languages, such as C++'s STL, etc. COW is a commonly used optimization method and can be classified into: delayed allocation of resources. Resources are only occupied when they are actually needed. Copy-on-write can usually reduce resource usage.
An example that proves that PHP COW optimizes memory usage:
<?php
$j = 1;
var_dump(memory_get_usage());
$tipi = array_fill(0, 100000, 'php-internal');
var_dump(memory_get_usage());
$tipi_copy = $tipi;
var_dump(memory_get_usage());
foreach ($tipi_copy as $i) {
$j += count($i);
}
var_dump(memory_get_usage());Running results:
$ php t . phpint(630904)int(10479840)int(10479944)int(10480040)
The memory is not significantly improved.
The principle of "copy-on-write"
Sharing the same memory with multiple variables of the same value does save memory space, but the value of the variable will change. If in the above example , if the value pointing to the same memory changes (or may change), the changed value needs to be "separated". This "separation" operation is "copying".
In PHP, in order to distinguish whether the same zval address is shared by multiple variables, the Zend engine introduces two variables, ref_count and is_ref, for identification:
ref_count and is_ref are defined in the zval structure The
is_ref mark in the body is a mandatory reference by the user using &;
ref_count is a reference count, used to identify how many variables this zval is referenced, that is, the automatic reference of COW, when it is 0 will be destroyed;
Note: It can be seen that there is no difference in PHP's use of memory between $a=$b; and $a=&$b; (when the value does not change);
I believe everyone can also understand the implementation principle of COW in PHP: COW in PHP is implemented based on reference counting ref_count and is_ref. If there is an additional variable pointer, ref_count will be increased by 1, otherwise subtracted by 1, and destroyed when it is reduced to 0; the same Reason, if there is one more mandatory reference &, is_ref will be increased by 1, otherwise it will be subtracted by 1.
Related recommendations:
Summarize the example usage tutorial of copy-on-write
##Copy-on-write (Copy On Write) in PHP Code example sharing
PHP variable reference counter and copy-on-write
The above is the detailed content of Detailed explanation of PHP copy-on-write. For more information, please follow other related articles on the PHP Chinese website!
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
1663
14
1420
52
1315
25
1266
29
1239
24
How do you parse and process HTML/XML in PHP?
Feb 07, 2025 am 11:57 AM
This tutorial demonstrates how to efficiently process XML documents using PHP. XML (eXtensible Markup Language) is a versatile text-based markup language designed for both human readability and machine parsing. It's commonly used for data storage an
Explain JSON Web Tokens (JWT) and their use case in PHP APIs.
Apr 05, 2025 am 12:04 AM
JWT is an open standard based on JSON, used to securely transmit information between parties, mainly for identity authentication and information exchange. 1. JWT consists of three parts: Header, Payload and Signature. 2. The working principle of JWT includes three steps: generating JWT, verifying JWT and parsing Payload. 3. When using JWT for authentication in PHP, JWT can be generated and verified, and user role and permission information can be included in advanced usage. 4. Common errors include signature verification failure, token expiration, and payload oversized. Debugging skills include using debugging tools and logging. 5. Performance optimization and best practices include using appropriate signature algorithms, setting validity periods reasonably,
Explain late static binding in PHP (static::).
Apr 03, 2025 am 12:04 AM
Static binding (static::) implements late static binding (LSB) in PHP, allowing calling classes to be referenced in static contexts rather than defining classes. 1) The parsing process is performed at runtime, 2) Look up the call class in the inheritance relationship, 3) It may bring performance overhead.
PHP Program to Count Vowels in a String
Feb 07, 2025 pm 12:12 PM
A string is a sequence of characters, including letters, numbers, and symbols. This tutorial will learn how to calculate the number of vowels in a given string in PHP using different methods. The vowels in English are a, e, i, o, u, and they can be uppercase or lowercase. What is a vowel? Vowels are alphabetic characters that represent a specific pronunciation. There are five vowels in English, including uppercase and lowercase: a, e, i, o, u Example 1 Input: String = "Tutorialspoint" Output: 6 explain The vowels in the string "Tutorialspoint" are u, o, i, a, o, i. There are 6 yuan in total
What are PHP magic methods (__construct, __destruct, __call, __get, __set, etc.) and provide use cases?
Apr 03, 2025 am 12:03 AM
What are the magic methods of PHP? PHP's magic methods include: 1.\_\_construct, used to initialize objects; 2.\_\_destruct, used to clean up resources; 3.\_\_call, handle non-existent method calls; 4.\_\_get, implement dynamic attribute access; 5.\_\_set, implement dynamic attribute settings. These methods are automatically called in certain situations, improving code flexibility and efficiency.
PHP and Python: Comparing Two Popular Programming Languages
Apr 14, 2025 am 12:13 AM
PHP and Python each have their own advantages, and choose according to project requirements. 1.PHP is suitable for web development, especially for rapid development and maintenance of websites. 2. Python is suitable for data science, machine learning and artificial intelligence, with concise syntax and suitable for beginners.
PHP in Action: Real-World Examples and Applications
Apr 14, 2025 am 12:19 AM
PHP is widely used in e-commerce, content management systems and API development. 1) E-commerce: used for shopping cart function and payment processing. 2) Content management system: used for dynamic content generation and user management. 3) API development: used for RESTful API development and API security. Through performance optimization and best practices, the efficiency and maintainability of PHP applications are improved.
PHP: A Key Language for Web Development
Apr 13, 2025 am 12:08 AM
PHP is a scripting language widely used on the server side, especially suitable for web development. 1.PHP can embed HTML, process HTTP requests and responses, and supports a variety of databases. 2.PHP is used to generate dynamic web content, process form data, access databases, etc., with strong community support and open source resources. 3. PHP is an interpreted language, and the execution process includes lexical analysis, grammatical analysis, compilation and execution. 4.PHP can be combined with MySQL for advanced applications such as user registration systems. 5. When debugging PHP, you can use functions such as error_reporting() and var_dump(). 6. Optimize PHP code to use caching mechanisms, optimize database queries and use built-in functions. 7
