PHP shared memory usage and signal control example analysis

This article mainly introduces the analysis of PHP shared memory usage and signal control examples. It has certain reference value. Now I share it with you. Friends in need can refer to it.

The examples in this article describe PHP sharing Memory usage and signal control. Share it with everyone for your reference, the details are as follows:

Shared memory

The use of shared memory is mainly to enable different users to use the same machine. Some data is shared among processes, such as the usage of the current process among multiple php-fpm processes. This kind of communication is also called Inter-Process Communication, or IPC for short.

PHP’s built-in shmop extension (Shared Memory Operations) provides a series of functions for shared memory operations (maybe because not many people use it, this document has not been translated into Chinese yet). On Linux, these functions are directly implemented by calling the shm* series of functions, while on Winodows, the same calls are also implemented by encapsulating system functions.

Main functions:

shmop_close — Close the shared memory block

shmop_delete — Delete the shared memory Block

shmop_open — Create or open a shared memory block

shmop_read — Read data from a shared memory block

shmop_size — Get the size of the shared memory block

shmop_write — Write data to the shared memory block

There is another important thing related to this Function: ftok, creates the unique key of IPC through the inode information of the file (viewed through stat or ls -i command on *nix) (the inode of the file/folder is unique). This function is also implemented by directly calling the system function of the same name on Linux, and some encapsulation is still used on Windows.

A simple counting example:

<?php
# 创建一块共享内存
$shm_key = ftok(__FILE__, &#39;t&#39;);
$shm_id = shmop_open($shm_key, &#39;c&#39;, 0644, 8);
# 读取并写入数据
$count = (int) shmop_read($shm_id, 0, 8) + 1;
shmop_write($shm_id, str_pad($count, 8, &#39;0&#39;, STR_PAD_LEFT), 0);
// echo shmop_read($shm_id, 0, 8);
# 关闭内存块，并不会删除共享内存，只是清除 PHP 的资源
shmop_close($shm_id);

The above code does not execute a count increase of 1, and the data is between different processes shared. In other words, unless this memory is manually deleted, this data will not be reset.

There is a point that needs a little attention: the second parameter of shmop_open is a flag, similar to the second parameter of fopen, and its values ​​​​are as follows:

"a" read-only Access;

"c" If the memory segment does not exist, create it, if it exists, it can be read and written;

"w" Read and write;

"n" Create If a new memory segment with the same key already exists, the creation will fail. This is for the sake of safe use of shared memory.

In addition, since the shared memory segment used is of fixed length, the length of the data must be calculated when storing and reading, otherwise the writing may fail or a null value may be read.

Signal Control

Since shared memory is used to store data above, you need to consider whether multiple processes are writing data to the shared memory at the same time. circumstances, whether conflicts need to be avoided. If so, you need to introduce a semaphore for control.

PHP also provides a similar built-in extension sysvsem (this extension is not available in the Windows environment. The ftok function is also included in this extension in the document, but in fact ftok is provided in the standard function library, so Also available under Windows).

Before talking about semaphore control, let me talk about another interesting thing: looking at the official documentation, you will find that there are also functions for shared memory operations (shm_*), because this is actually the same category (or Three extensions from the same author) and one is sysvmsg (queue messages). The implementation of the functions is slightly different, but what they actually do is basically the same. What is the difference between this and the shmop extension above? The README file under the shmop source code has a simple description:

PHP already had a shared memory extension (sysvshm) written by Christian Cartus 32982644a4ae8a6ce575374773bddd52, unfortunately this extension was designed with PHP only in mind and offers high level features which are extremely bothersome for basic SHM we had in mind.

To put it simply: the method provided by the sysvshm extension is not to store user data intact, but The parameters are first serialized using PHP's variable serialization function and then stored. This means that data stored through these methods cannot be shared with non-PHP processes. However, this can also store richer PHP data types. In the above extension, shmop_write can only write strings. So why doesn't sysvshm support Windows as well? Because it does not introduce the header file of tsrm_win32.h that encapsulates the shm* series of functions.

Example after introducing signal control:

<?php
$id_key = ftok(__FILE__, &#39;t&#39;);
$sem_id = sem_get($id_key);
# 请求信号控制权
if (sem_acquire($sem_id)) {
  $shm_id = shmop_open($id_key, &#39;c&#39;, 0644, 8);
  # 读取并写入数据
  $count = (int) shmop_read($shm_id, 0, 8) + 1;
  shmop_write($shm_id, str_pad($count, 8, &#39;0&#39;, STR_PAD_LEFT), 0);
  // echo shmop_read($shm_id, 0, 8);
  # 关闭内存块
  shmop_close($shm_id);
  # 释放信号
  sem_release($sem_id);
}

But it is actually very difficult to simulate write conflicts locally (considering the computer execution speed). In the local test, if you do not use shmop_close to close the resource when using a for loop operation, an error warning that the shared memory cannot be opened will appear. This may be caused by the shared memory being occupied by the previous operation and not being released yet.

Related recommendations:

shmop series functions use PHP shared memory implementation method

Detailed introduction to the code used by PHP shared memory

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