2.3.4. OPcache and JIT

OPcache is designed to cache compiled PHP script bytecode in RAM. Its use allows you to speed up the site scripts to some extent and reduce the load on the web server. OPcache can be used on any site, no special support is required.

1. Open the "PHP settings" section and switch to the "OPcache" tab.
2. In the "OPcache settings" block, in the "Allocated memory capacity" field, select the desired memory capacity and click "Save":
3. Wait approximately 15 minutes for the changes to take effect.

The parameter "Frequency of checking for changes to files" (opcache.revalidate_freq) is zero by default. This means that each request will check if the script has changed and can still be fetched from the cache or must be called again. Using a value greater than zero makes sense if the site receives a large number of requests and its files are not edited. This will give some performance gain by reducing the number of disk operations.

Viewing statistics:

1. Open the "OPcache" section.
2. In the "Sites with enabled OPcache" block, in the row with the desired site, click "Statistics":

Example statistics:

• Status
• Configuration
• Scripts

Chart "Percentage of total volume" shows:

• Percentage of used memory.
• Percentage of free memory.
• Percentage of wasted memory. Wasted memory is memory that is used by data that is already obsolete and irrelevant. It appears due to changes in scripts or impossibility to use current data for them. Such memory is cleared automatically when a certain share of the maximum of the total memory is reached, so manual clearing is not required. This approach ensures faster operation of OPcache.

Chart "Percentage of all requests" shows:

• Hits — statistics of requests to scripts that were in the cache and the result was provided without executing the source code.
• Misses — statistics of script requests that not were in the cache and were executed.

A complete list of OPcache options is available in the official documentation.

List of OPcache options and their current values:

Information about scripts and their optimization:

1. Open the "PHP settings" section and switch to the "OPcache" tab.
2. In the "OPcache settings" block, in the "Allocated memory capacity" field, select "Disabled" and click "Save":
3. Wait approximately 15 minutes for the changes to take effect.

JIT (Just-In-Time) works in combination with OPcache and compiles code into processor instructions directly at runtime, so that some code is executed without a Zend virtual machine at all and is passed directly to the processor for execution. This allows the code to execute even faster than with OPcache without JIT.

The effect of using JIT is more noticeable with code that performs active calculations and intensively uses processor resources, and less noticeable with a large number of I/O operations.

1. Enable OPcache (if not enabled).
2. Open the "PHP settings" section and switch to the "OPcache" tab.
3. In the "OPcache settings" block in the "OPcache JIT" section, in the "OPcache JIT" field, select the desired memory capacity, settings and click "Save":
4. Wait approximately 15 minutes for the changes to take effect.

You can check the status of the JIT with a simple PHP script:

<?php
var_dump(opcache_get_status()['jit']);

The result will be something like this:

array(7) {
  ["enabled"]=>
  bool(true)
  ["on"]=>
  bool(true)
  ["kind"]=>
  int(5)
  ["opt_level"]=>
  int(5)
  ["opt_flags"]=>
  int(6)
  ["buffer_size"]=>
  int(33554416)
  ["buffer_free"]=>
  int(33551984)
}

If the enabled and on values are both true, then all is well.

For JIT, you can configure the use of CPU optimization, register allocation, compilation trigger conditions (trigger), and optimization level:

The most optimal value is considered to be 1255 (as in the screenshot above), which allows the JIT to be maximized.

1. Open the "PHP settings" section and switch to the "OPcache" tab.
2. In the "OPcache settings" block, in the "OPcache JIT" section, select "Disabled" in the "Allocated memory capacity" field and click "Save":
3. Wait approximately 15 minutes for the changes to take effect.