Mastering PHP Shared Memory: Fast IPC Techniques and Performance Benchmarks

Usage Scenario

For a single machine, errors from a specific host can be aggregated and alerted; identical alerts within a minute are merged using shared memory for temporary storage, providing an efficient solution.

PHP Session

If a service runs on a single server with sessions enabled, replacing file‑based sessions with shared memory can significantly improve speed, though it lacks the flexibility of memcache.

What Is Shared Memory

Shared memory allows different processes on the same machine to exchange data by creating a memory segment identified by a unique shmid . Processes can read, write, or delete data in this segment without kernel involvement, making it a fast form of inter‑process communication (IPC).

Common PHP Shared Memory Methods

APC can cache PHP opcode and share data among PHP‑FPM processes (functions: apc_store , apc_fetch , apc_add , etc.).

Shmop (Unix) provides shmop_open , shmop_close , shmop_read , shmop_write , shmop_delete .

System V shared memory functions include ftok , shm_attach , shm_detach , shm_put_var , shm_get_var , shm_remove_var .

Semaphores ( sem_get , sem_acquire , sem_release ) are needed to ensure atomic operations.

Creating a Memory Segment

<code>&lt;?php
$key = ftok(__FILE__, 'h');
$mode = 'c';
$permissions = 0644;
$size = 1024;
$shmid = shmop_open($key, $mode, $permissions, $size);
?&gt;</code>

The first argument is a key generated by ftok . The mode follows fopen conventions ( a read‑only, w read‑write, c create or open, n create only). Permissions are an octal value, and size is in bytes.

Writing Data

<code>&lt;?php
$shmid = shmop_open(ftok(__FILE__, 'h'), 'c', 0644, 1024);
shmop_write($shmid, "Hello World!", 0);
?&gt;</code>

The function takes the shared memory ID, the data string, and the offset (usually 0). It returns the number of bytes written or FALSE on failure.

Reading Data

<code>&lt;?php
$shmid = shmop_open(ftok(__FILE__, 'h'), 'c', 0644, 1024);
var_dump(shmop_read($shmid, 0, 11));
?&gt;</code>

Parameters are the shared memory ID, start offset (0 for the beginning), and the number of bytes to read (often shmop_size($shmid) ).

Deleting a Memory Segment

<code>&lt;?php
$shmid = shmop_open(ftok(__FILE__, 'h'), 'c', 0644, 1024);
shmop_delete($shmid);
?&gt;</code>

Deletion marks the segment for removal; it is actually removed after all processes detach.

Closing a Memory Segment

<code>&lt;?php
$shmid = shmop_open(ftok(__FILE__, 'h'), 'c', 0644, 1024);
shmop_write($shmid, "Hello World!", 0);
shmop_delete($shmid);
shmop_close($shmid);
?&gt;</code>

After operations, the segment must be closed with shmop_close .

Atomic Operations – Semaphore Control

Write operations are not atomic; to handle concurrent access, use semaphores:

<code>&lt;?php
$key = ftok(__FILE__, 'h');
$semid = sem_get($key);
if (sem_acquire($semid)) {
    $shmid = shmop_open($key, 'c', 0644, 1024);
    shmop_write($shmid, '13800138000', 0);
    shmop_close($shmid);
    sem_release($semid);
}
?&gt;</code>

Performance Comparison

Reading/writing 1 KB of data 100 000 times shows the following average times (seconds):

Memcache – read 7.8, write 8.11

File – read 2.6, write 3.2

Shared memory – read 0.1, write 0.07

The results demonstrate that shared memory provides the fastest I/O among the three methods.