How to limit memory usage of PHP process using memory_limit

Memory_limit in virtual host PHP is the maximum available memory limit for a single execution of a single PHP script; the default limit is 256MB, and the maximum can be adjusted to 512MB. The editor below introduces how to use memory_limit to limit the memory usage of the PHP process.

memory_limit As the name suggests, it limits the memory usage of the PHP process. For example:

magento2’s system requirements have restrictions on PHP memory_limit, which cannot be lower than 512M. (The default value is 128M. If it is not changed, magento’s background processing logic will not be executed normally)

Look at the explanation on the PHP official website

This sets the maximum amount of memory in bytes that a script is allowed to allocate. This helps prevent poorly written scripts for eating up all available memory on a server. Note that to have no memory limit, set this directive to -1.

Required Note that the higher the value of memory_limit, that is, the more memory a single PHP process takes up, the fewer requests the system can handle concurrently. For example, if a machine with 2G memory

memory_limit is set to 128M, it can handle up to 16 requests at the same time. If memory_limit is set to 256M, it can handle up to 8 requests at the same time. If memory_limit is set to 512M, it can handle up to 4 requests at the same time. Request

Is the larger the value of memory_limit, the better?

Of course not, memory_limit is mainly to prevent program bugs or infinite loops from occupying a large amount of memory, causing system downtime. When introducing a large number of third-party plug-ins or codes, it is very necessary to limit memory.

Memory_limit will make each PHP process occupy a fixed amount of memory?

Or is it just the upper limit of allocated memory? Test the

idea, set memory_limit to 10M, initialize a 2M/20M string in the PHP request, and see the memory usage in the system process.

Nginx configuration

server {
        listen 8093;
        root /home/zhongwei/work/test/memory_limit/;
        index index.php index.html index.htm;
        server_name localhost;
        location / {
                # try_files $uri $uri/ =404;
                try_files $uri $uri/ /index.php?q=$uri&$args;
        }
        location ~ \.php$ {
include snippets/fastcgi-php.conf;
fastcgi_param PHP_VALUE "memory_limit = 10M";
fastcgi_pass unix:/run/php/php7.0-fpm.sock;
        }
}

PHP test file

<?php
$char_count = 2;
$M = 1024 * 1024;
echo sprintf("Current memory_limit value is: %s.", ini_get(&#39;memory_limit&#39;));
echo sprintf(&#39;<br/>Amount of memory allocated to PHP: %0.3fM.&#39;, memory_get_usage() / $M);
$s = str_repeat("a", $M * $char_count);
//sleep(30);
echo sprintf(&#39;<br/>Amount of memory allocated to PHP: %0.3fM.&#39;, memory_get_usage() / $M);
echo sprintf(&#39;<br/>Total memory allocated from system: %0.3fM.&#39;, memory_get_usage($real_usage=true) / $M);
echo &#39;<br/>success&#39;;

Test result

When $char_count is 2, a string occupying 2M of memory is initialized and output The result is

Current memory_limit value is: 10M.
Amount of memory allocated to PHP: 0.344M.
Amount of memory allocated to PHP: 2.348M.
Total memory allocated from system: 4.004M.
success

$char_count is 20, that is, a string occupying 20M of memory is initialized, and the output result is

Current memory_limit value is: 10M.
Amount of memory allocated to PHP: 0.346M.

Note that the HTTP status code is 500, which means that the string is executed During initialization, the PHP process was killed by the system.

Look at the log information recorded in Nginx error.log

2017/03/01 10:41:23 [error] 6903#6903: *39 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Allowed memory size of 10485760 bytes exhausted (tried to allocate 20971552 bytes) in /home/zhongwei/work/test/memory_limit/index.php on line 8
PHP message: PHP Stack trace:
PHP message: PHP   1. {main}() /home/zhongwei/work/test/memory_limit/index.php:0
PHP message: PHP   2. str_repeat() /home/zhongwei/work/test/memory_limit/index.php:8" while reading response header from upstream, client: 127.0.0.1, server: localhost, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.0-fpm.sock:", host: "localhost:8093"

The actual test results show that memory_limit only limits the upper limit of memory usage for each PHP process, rather than allocating it to each process. Fixed memory. Therefore, the number of concurrency will not decrease because the memory_limit setting is larger.

What is the default value of memory_limit

• Before PHP 5.2 it was 8M

• PHP 5.2 is 16M

• The version after PHP 5.2 is 128M

Recommended learning: php video tutorial

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