Caching explained

Learn all about the different cache types available for PHP, websites, and WordPress in particular.

Zend Opcode, JIT & WP Object Cache

OPcache

OPcache enhances PHP performance by archiving pre-compiled script bytecode in shared memory, which eliminates the need for PHP to load and parse scripts on each request. It also provides a special buffer to store immutable strings so that they can be reused by their pointers for all occurrences of the same string.


Advantage	Major performance improvement.
Activation	Must be enabled in the php.ini configuration file (opcache. enable, opcache. memory_consumption).
Suggestion	Memory min. 128 MB. String buffer min. 8MB.

Use the atec-Cache-Info plugin to check the OPcache options, usage and statistics.

JIT

JIT can bring performance improvements by compiling and storing the full or frequently called parts of a PHP application as CPU machine code, and directly execute it, which bypasses the Zend VM and its process overhead.


Advantage	Performance improvement.
Activation	Must be enabled in the php.ini configuration file (opcode. jit, opcache. jit_buffer_size) and requires Apache to be enabled.
Suggestion	Enabled with small memory size, 2 MB should be enough.
Downside	Can cause unpredictable effects.

Use the atec-Cache-Info plugin to check the OPcache options, usage and statistics.

WP Object Cache

The WordPress Object Cache is used to save on trips to the database. The Object Cache stores all of the cache data to memory and makes the cache contents available by using a key, which is used to name and later retrieve the cache contents.

The WP Object Cache is a volatile cache, that is only valid for the current request.


Advantage	Is a standard class in WordPress.
Activation	Enabled by default. No configuration.
Suggestion	Use a persistent object cache to improve performance.

Use the atec-Cache-Info plugin to check the OPcache options, usage and statistics.

Persistent Object Cache

Persistent caches store the cached data in a persistent storage so that it can be re-used multiple times without refreshing the original data. It is technically possible to use a database for storage, but memory storage is the only performant option.


Advantage	Major performance improvement for front and back end.
Activation	Each storage option requires it's own extension and the service must be enabled on the server.
Suggestion	32M is a good value to start with. Increase it if not enough.

APCu Cache

APCu (APC User Cache) is a user-level caching solution for PHP that caches data in the server's memory. APCu is the fastest persistent object cache available. It is the only cache that can store objects without serialization.

Use the atec-Cache-APCu plugin to enable object caching.

Redis Cache

Redius is a fast in-memory database using the server's shared memory. If you have multiple WordPress installed in a shared hosting account, all of these installations share the same memory.

Storing non-scalar data in a database requires serialization, which is an extra step when storing/retrieving data. Therefore Redis is slower than APCu but faster than no-cache. Memcached can be accessed via TCP/IP or UNIX socket – which is the faster of the two.

Use the atec-Cache-Redis plugin to enable object caching.

Memcached Cache

Memcached is an in-memory key-value store for small chunks of arbitrary data (strings, objects) from results of database calls, API calls, or page rendering.

Storing non-scalar data in a database requires serialization, which is an extra step when storing/retrieving data. Therefore Memcached is slower than APCu but faster than no-cache. Redis can be accessed via TCP/IP or UNIX socket – which is the faster of the two.

Use the atec-Cache-Memcached plugin to enable object caching.

Browser Cache & Content Delivery Networks (CDN)

Browser Cache

To shorten page load times, browsers cache most of the content that appears on a web page, saving a copy of the web page's content on the device's hard drive. This way, the next time the user loads the page, most of the content is already stored locally and the page will load much more quickly.


Advantage	Saving load time and bandwidth.
Activation	Browser caching is enabled by default, but can be improved by cache directives on the server. The settings can either be set in the web-server configuration or in the .htaccess file (Apache web-server).
Suggestion	Set the Cache-Control header with the a high max-age value.

Use the atec-htaccess plugin to set the „cache-control header“.

CDN

A content delivery network (CDN) is a distributed group of servers that caches content near end users.


Advantage	Reduces the load on a server and improves the response time to the client.
Activation	Requires a subscription with a CDN-provider such as cloudflare or bunny.net.
Suggestion	A CDN is not required for small static sites with a local user base, but can drastically improve load times for a global user base, as the CDN provides lower latency for every request.

Use the atec-page-performance and atec-profiler plugin to measure your site performance and server load.