A preliminary study on multiple processes in PHP7

Preparation

We all know that PHP is executed in a single process. PHP's handling of multi-concurrency mainly relies on the multi-process of the server or PHP-FPM and the reuse of their processes, but PHP Implementing multiple processes is also of great significance, especially when processing large amounts of data in background Cli mode or running background DEMON daemon processes. Needless to say, the advantages of multiple processes.

PHP's multi-threading has also been mentioned, but the problem of multi-thread resource sharing and allocation within the process is difficult to solve. PHP also has multi-threading extension pthreads , but it is said to be unstable and requires the environment to be thread-safe, so it is not used much.

In the past, a great master in the PHP group once guided that if you want to advance in background PHP, you must avoid multi-process. It just so happens that the daemon process in the company also uses PHP's multi-process, combined with Gu Ge's various After reading the information and manuals, I finally understood multi-processing and wrote a small demo (implemented on a Linux system). I will summarize it in this article. If there are any mistakes or omissions, thank you for mentioning them.

To implement PHP multi-process, we need two extensions pcntl and posix. The installation method will not be described here.

In php we use pcntl_fork() to create multiple processes (in C language programming of *NIX systems, existing processes generate new processes by calling the fork function). The new process after the fork becomes the child process, the original process becomes the parent process, and the child process has a copy of the parent process. Note here:

• The child process shares the program text segment with the parent process

• The child process has a copy of the parent process's data space and heap and stack. Note that it is a copy, not a share

• The parent process and the child process will continue to execute the program code after the fork

• After the fork, whether the parent process or the child process executes first cannot be confirmed, and it depends on the system scheduling (depends on belief)

It is said here that the child process has a copy of the parent process's data space, heap, and stack. In fact, in most implementations, it is not a true complete copy. More importantly, COW (Copy On Write) technology is used to save storage space. To put it simply, if neither the parent process nor the child process modifies these data, heap, and stack, then the parent process and the child process temporarily share the same data, heap, and stack. Only when the parent process or child process attempts to modify the data, heap, and stack will a copy operation occur. This is called copy-on-write.

After calling pcntl_fork(), this function will return two values. Returns the process ID of the child process in the parent process, and returns the number 0 inside the child process itself. Since multiple processes cannot run properly in the apache or fpm environment, you must execute the code in the php cli environment.

Create sub-process

Creating a PHP sub-process is the beginning of multiple processes, we need pcntl_fork() function;

Detailed explanation of fork function

pcntl_fork() — Generate a branch (child process) at the current position of the current process. After this function creates a new child process, the child process will inherit the current context of the parent process and continue to execute downwards from the pcntl_fork() function like the parent process, except that the pcntl_fork is obtained () The return value is different, we can distinguish the parent process and the child process by judging the return value, and allocate the parent process and the child process to do different logical processing.

When the pcntl_fork() function is successfully executed, it will return the process id (pid) of the child process in the parent process. Because the pid of the system's initial process init process is 1, the pid of the subsequent processes will be greater than this process, so We can confirm that the current process is the parent process by judging that the return value of pcntl_fork() is greater than 1; in the child process, the return value of this function will be a fixed value of 0. We can also judge that the return value of pcntl_fork() is 0. To determine the child process; when the pcntl_fork() function fails to execute, it will return -1 in the parent process, and of course no child process will be generated.

fork process instance

fork child process

$ppid = posix_getpid();
$pid = pcntl_fork();
if ($pid == -1) {
    throw new Exception('fork child process fail');
} elseif ($pid > 0) {
    cli_set_process_title("我是父 process，pid is : {$ppid}.");
    sleep(30);
} else {
    $cpid = posix_getpid();
    cli_set_process_title("我是 {$ppid} 子的 process,我的 process pid is : {$cpid}.");
    sleep(30);
}

Description:

posix_getpid(): Return the current process id

cli_set_process_title('Process Name'): Give the current process a loud name.

Running this example, we can see the current two PHP processes.

www@iZ2zec3dge6rwz2uw4tveuZ:~/test$ ps aux|grep -v grep |grep 我
www      18026  0.5  1.2 204068 25772 pts/0    S+   14:08   0:00 我是父 process，pid is : 18026.
www      18027  0.0  0.3 204068  6640 pts/0    S+   14:08   0:00 我 18026 子的 process,我的 process pid is : 18027.　

The first section of code, after the program starts from pcntl_fork(), the parent process and the child process will continue to execute the code:

$pid = pcntl_fork();
if( $pid > 0 ){
  echo "我是父亲".PHP_EOL;
} else if( 0 == $pid ) {
  echo "我是儿子".PHP_EOL;
} else {
  echo "fork失败".PHP_EOL;
}　

The result:

www@iZ2zec3dge6rwz2uw4tveuZ:~/test$ php 123.php
我是父亲
我是儿子

The second section Code to illustrate that the child process has a copy of the data of the parent process, rather than sharing:

// 初始化一个 number变量 数值为1
$number = 1;
$pid = pcntl_fork();
if ($pid > 0) {
    $number += 1;
    echo "我是父亲，number+1 : { $number }" . PHP_EOL;
} else if (0 == $pid) {
    $number += 2;
    echo "我是儿子，number+2 : { $number }" . PHP_EOL;
} else {
    echo "fork失败" . PHP_EOL;
}

Result

www@iZ2zec3dge6rwz2uw4tveuZ:~/test$ php 1234.php
我是父亲，number+1 : { 2 }
我是儿子，number+2 : { 3 }

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