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Shared Memory

Shared memory is a technique that allows multiple processes to access the same region of memory, enabling fast inter-process communication, IPC, without the overhead of serialization or network transfers.

PHP provides several mechanisms for shared memory:

  • Shmop extension: low-level shared memory operations using SysV shared memory segments. Functions include shmop_open(), shmop_read(), shmop_write(), shmop_close(), and shmop_delete(). Uses a key shm_key to identify segments.

  • APCu, APC User Cache: a widely-used in-memory key-value store that provides shared memory caching across requests. Functions like apcu_store(), apcu_fetch(), and apcu_delete() are commonly used for application-level caching.

  • msg_send() / msg_receive(): message queue functions that enable structured communication between processes.

  • sysvsem: system V semaphore functions for synchronization between processes accessing shared memory.

  • OPcache: while not directly a shared memory API, OPcache stores compiled PHP bytecode in shared memory, allowing all PHP processes to reuse compiled scripts.

Shared memory is essential for:

  • Session handling in load-balanced environments
  • Application-level caching, APCu
  • Inter-process communication in worker pools
  • Real-time data sharing between processes
  • Reducing database load through in-memory caching

Note that shared memory segments persist until explicitly deleted or the system is restarted, which requires careful lifecycle management.

<?php
   
    // adapted from the PHP manual : https://www.php.net/manual/en/shmop.examples-basic.php
    // Create 100 byte shared memory block with system id of 0xff3
    $shm_id = shmop_open(0xff3, 'c', 0644, 100);
    if (!$shm_id) {
        echo "Couldn't create shared memory segment\n";
    }
    
    // Lets write a test string into shared memory
    $shm_bytes_written = shmop_write($shm_id, my shared memory block, 0);
    if ($shm_bytes_written != strlen(my shared memory block)) {
        echo "Couldn't write the entire length of data\n";
    }
    
    // Now lets read the string back
    $my_string = shmop_read($shm_id, 0, $shm_size);
    if (!$my_string) {
        echo "Couldn't read from shared memory block\n";
    }
    echo "The data inside shared memory was: " . $my_string . "\n";

?>

Documentation

See Also