PHP opcode Caching principle

What is a opcode cache?

When the interpreter finishes parsing the script code, it generates intermediate code that can be run directly, also known as the opcode (Operate code,opcode). The goal of the Opcode cache is to avoid duplication of compilation and reduce CPU and memory overhead. If the performance bottleneck for dynamic content is not CPU and memory, but I/O operations, such as disk I/O overhead from database queries, the performance gains of the opcode cache are very limited. But since the opcode cache can bring down CPU and memory overhead, this is always a good thing.

Modern opcode buffers (optimizer+,apc2.0+, others) are stored using shared memory and can be executed directly from the file without the "deserialization" code before execution. This leads to significant performance acceleration, which typically reduces the overall server's memory consumption and has few drawbacks.

Why use the opcode cache?

This has to be said from the life cycle of the PHP code, which takes five steps when requesting a PHP script, as shown in:

The Zend engine must read files from the file system, scan its dictionaries and expressions, parse files, create computer code to execute (called opcode), and finally execute opcode. Each time the PHP script will execute the above steps, if the PHP source code does not change, then opcode will not change, obviously there is no need to re-generate opcode every time, combined with the ubiquitous caching mechanism in the web, we can cache opcode, It is not faster to access the cached opcode directly later, and the flowchart after enabling opcode caching is as follows:

are there those php opcode cache plugins?

Optimizer+ (optimizer+ in mid-March 2013 opcache,php 5.5 episode Opcache, the others will not disappear? ), Eaccelerator, XCache, APC ...

PHP opcode principle

OpCode is a PHP script-compiled intermediate language, like Java's bytecode, or. NET MSL, for example, you write down the following PHP code:

<?php   echo "Hello World";   $a = 1 + 1;   echo $a;? >

　　

PHP executes this code in the following 4 steps (to be exact, it should be PHP's language engine Zend)

1. Scanning (lexing), converting the PHP code to a language fragment (Tokens)
2. parsing, converting tokens into simple and meaningful expressions
3. compilation, compile the expression into Opocdes
4. Execution, execute opcodes sequentially, one at a time, thus realizing the function of PHP script

Aside: Now some caches such as APC, can make PHP cache opcodes, so that every time there is a request, there is no need to repeat the previous 3 steps, so that can greatly improve the speed of PHP execution.

What is lexing? Students who have learned the principles of compiling should have some knowledge of the lexical analysis steps in the compiling principle, and Lex is the basis table for lexical analysis. ZEND/ZEND_LANGUAGE_SCANNER.C will perform lexical analysis based on the PHP code entered by the ZEND/ZEND_LANGUAGE_SCANNER.L (lex file) to get a "word", PHP4.2 began to provide a function called Token_get_all, this function can be said a section of PHP code scanning into tokens;

The identification of the opcode number, indicating the type of operation for each op_array, such as add, the echo result holds opcode result operand 1 to opcode operand operand 2 extended value 1 shaping to distinguish the overloaded operator

* zend_echo ' Hello world ' * zend_add ~0 1 1* zend_assign!0 ~0* zend_echo!0

You might ask, where is our $ A?

This introduces the operands, each of which consists of the following two parts:

1. Op_type: For Is_const, Is_tmp_var, Is_var, is_unused, or IS_CV b)
2. U, a consortium, holds the value (const) or Lvalue (Var) of this operand in different types, depending on the op_type.

And for Var, every Var is different.

Is_tmp_var, as the name implies, this is a temporary variable, save some Op_array results, so that the next op_array to use, this operand of u holds a pointer to the variable table of a handle (integer), this operand is generally used to start, such as ~0, A temporary variable that represents the unknown number No. 0 of the variable table

Is_var This is our general sense of the variable, they start with a $ expression

IS_CV says ZE2.1/ PHP5.1 later compiler uses a cache mechanism, this variable holds the address of the variable referenced by it, when a variable is referenced for the first time, it will be CV up, the reference to this variable will not need to find the active symbol table again, CV variable to! The beginning indicates.

So it seems that our $ A is optimized to 0.

PHP opcode Caching principle