Profiling Zend Opcache fragmentation latency

Hardware and Baseline Environment State

The infrastructure consists of a localized cluster of three physical nodes. The primary application node utilizes dual AMD EPYC 7443 processors (48 cores, 96 threads per socket), 256GB of DDR4 ECC memory operating at 3200MHz, and a storage array of four 1.92TB enterprise NVMe solid-state drives configured in a software RAID 10 matrix. The operating system is Debian 12 (Bookworm) running the unmodified 6.1.0-18-amd64 kernel.

The software stack operates on Nginx 1.24.0, functioning as the primary web server and reverse proxy, passing requests to PHP 8.2.10 via the FastCGI Process Manager (FPM) protocol. The persistence layer is handled by PostgreSQL 15, and Redis 7 handles transient object caching. The hosted application is a corporate service portal utilizing the Arotech - Technology IT Services WordPress Theme. The platform serves a steady, predictable baseline of requests primarily localized to regular business hours.

The Degradation Indicator

At 02:00 UTC, system monitoring dashboards recorded a shift in the response time percentiles. While the 50th percentile (P50) latency remained stable at 45 milliseconds, the 99th percentile (P99) latency exhibited intermittent, severe elongations, stretching from a baseline of 120 milliseconds to exceeding 850 milliseconds. CPU utilization across all 192 logical cores remained below 4%. Memory utilization was static. Network throughput was negligible.

These elongations occurred consistently during the system's quietest operational window. The symptom isolated itself from user-generated HTTP traffic. It correlated precisely with the execution of a background cron routine initiated by the local crond daemon, which executed a PHP script via the command line interface to compile and cache routine data aggregations. The delay was not localized to the cron script itself; rather, while the cron script executed, incoming HTTP requests processed by the PHP-FPM worker pool experienced the 850-millisecond latency penalty.

The application architecture relies heavily on dynamic file inclusion routines standard in modern PHP frameworks. The backend service execution environments, especially when administrators Download WordPress Themes built with complex dependency injection containers, require the PHP Zend Engine to parse, compile, and execute thousands of individual PHP files per request. To mitigate the overhead of lexical analysis and compilation, the Zend Opcache extension is enabled. Opcache stores precompiled script bytecode in shared memory, eliminating the need for PHP to load and parse scripts on subsequent requests.

CPU Profiling with perf

The discrepancy between low resource utilization and high latency pointed toward a locking mechanism or a synchronization stall within the PHP-FPM processes. To observe the execution state of the processor instructions during the latency spikes, I utilized the Linux perf utility.

I initiated a system-wide profile capture targeting all CPUs for a duration of 60 seconds, encompassing the 02:00 UTC execution window.

perf record -F 99 -a -g -- sleep 60

The -F 99 parameter sets the sampling frequency to 99 Hertz, a value chosen to avoid lockstep with standard 100 Hertz system timer ticks, thereby preventing sampling bias. The -a flag profiles all processors, and -g captures the call graphs.

After the capture completed, I analyzed the resultant perf.data file.

perf report --stdio --no-children

The output revealed a highly concentrated execution overhead entirely disconnected from database queries or network wait states.

# Overhead  Command          Shared Object               Symbol
# ........  ...............  ..........................  ......................................
#
    34.12%  php-fpm          opcache.so                  [.] zend_shared_alloc_lock
    22.45%  php-fpm          opcache.so                  [.] zend_shared_alloc_unlock
    14.05%  php-fpm          opcache.so                  [.] accel_move_string
     8.12%  php            opcache.so                  [.] zend_accel_hash_update
     4.33%  libc.so.6        libc.so.6                   [.] __memcpy_avx_unaligned
     2.10%  php-fpm          php-fpm                     [.] zend_string_alloc
     1.45%  [kernel.kallsyms] [kernel.kallsyms]          [k] _raw_spin_lock

More than 56% of the sampled CPU time within the PHP processes was consumed by two functions within the opcache.so shared object: zend_shared_alloc_lock and zend_shared_alloc_unlock.

The Zend Opcache allocates a specific segment of RAM as shared memory (SHM). This shared memory segment allows multiple isolated PHP-FPM worker processes, as well as separate PHP CLI processes, to access the exact same precompiled opcode arrays without duplicating memory. Because multiple independent processes are reading from and potentially writing to this singular memory segment simultaneously, Opcache employs a system-level locking mechanism to prevent data corruption.

When a PHP script requests a file, Opcache checks its hash table. If the file is not cached (a cache miss), the Zend Engine compiles the file into opcodes. To store these new opcodes in the shared memory, the process must acquire an exclusive lock on the entire Opcache shared memory segment.

The perf data indicated that FPM workers were stalling because they were waiting to acquire the shared memory lock. The process holding the lock was the PHP CLI process executing the background cron routine. The cron script was compiling new files and writing to the Opcache, locking out the FPM workers serving live HTTP traffic.

Opcache Shared Memory and Interned Strings Analysis

To determine why the cron script was inducing heavy write operations to the Opcache during a period when all application files should theoretically have been cached hours prior, I needed to inspect the internal state of the Opcache memory segment.

Opcache allocates its memory into several distinct pools. The primary pool stores the compiled zend_op_array structures representing the executable code. Another critical pool is the Interned Strings Buffer.

String interning is a memory optimization technique. In a typical PHP application, identical strings (such as variable names, function names, and array keys) appear thousands of times across hundreds of files. Instead of storing a separate copy of the string "WP_Query" for every file that references it, the Zend Engine stores a single instance of the string in the Interned Strings Buffer. Every subsequent use of "WP_Query" simply points to this single memory address. This significantly reduces overall memory consumption.

I wrote a targeted PHP script to extract the raw telemetry from the Opcache extension. Instead of utilizing web-based graphical interfaces which introduce their own HTTP overhead, I executed this script via the command line to dump the memory status arrays directly into a JSON format for inspection.

 [
        'used' => round($memory_usage['used_memory'] / 1024 / 1024, 2) . ' MB',
        'free' => round($memory_usage['free_memory'] / 1024 / 1024, 2) . ' MB',
        'wasted' => round($memory_usage['wasted_memory'] / 1024 / 1024, 2) . ' MB',
        'current_wasted_percentage' => $memory_usage['current_wasted_percentage'] . '%'
    ],
    'interned_strings' => [
        'buffer_size' => round($interned_strings_usage['buffer_size'] / 1024 / 1024, 2) . ' MB',
        'used_memory' => round($interned_strings_usage['used_memory'] / 1024 / 1024, 2) . ' MB',
        'free_memory' => round($interned_strings_usage['free_memory'] / 1024 / 1024, 2) . ' MB',
        'number_of_strings' => $interned_strings_usage['number_of_strings']
    ],
    'statistics' => [
        'num_cached_scripts' => $opcache_statistics['num_cached_scripts'],
        'num_cached_keys' => $opcache_statistics['num_cached_keys'],
        'max_cached_keys' => $opcache_statistics['max_cached_keys'],
        'oom_restarts' => $opcache_statistics['oom_restarts'],
        'hash_restarts' => $opcache_statistics['hash_restarts'],
        'manual_restarts' => $opcache_statistics['manual_restarts'],
        'misses' => $opcache_statistics['misses'],
        'hits' => $opcache_statistics['hits']
    ]
];

echo json_encode($output, JSON_PRETTY_PRINT) . PHP_EOL;

I executed the script.

php /var/tmp/opcache_dump.php

{
    "memory": {
        "used": "112.45 MB",
        "free": "14.11 MB",
        "wasted": "1.44 MB",
        "current_wasted_percentage": "1.125%"
    },
    "interned_strings": {
        "buffer_size": "8 MB",
        "used_memory": "7.99 MB",
        "free_memory": "0.01 MB",
        "number_of_strings": 142051
    },
    "statistics": {
        "num_cached_scripts": 4102,
        "num_cached_keys": 6014,
        "max_cached_keys": 16229,
        "oom_restarts": 0,
        "hash_restarts": 0,
        "manual_restarts": 0,
        "misses": 8412,
        "hits": 4819202
    }
}

The output isolated the exact subsystem failure point. The total Opcache memory allocation was 128 MB (calculated from used + free + wasted). The primary memory pool had 14.11 MB of free space. The script limit max_cached_keys was set to 16,229, and only 6,014 keys were utilized.

However, the interned_strings subset was completely exhausted. The buffer_size was fixed at 8 MB. The used_memory was 7.99 MB. The free_memory was 0.01 MB.

Mechanism of Interned String Buffer Exhaustion

When the Zend Engine encounters a string literal during the compilation of a PHP file, it attempts to store that string in the Interned Strings Buffer within the Opcache shared memory.

If the Interned Strings Buffer is full, the system does not crash, nor does it immediately trigger an Out Of Memory (OOM) restart of the entire Opcache (as indicated by the oom_restarts counter remaining at 0). Instead, the Zend Engine falls back to allocating the memory for that specific string in the local process memory of the active worker, rather than in the shared memory segment.

This fallback mechanism introduces a severe performance penalty. When the background cron script executed at 02:00 UTC, it loaded application files and framework components. Because the Interned Strings Buffer was full, it could not intern new strings globally. Furthermore, strings allocated in local worker memory are not shared.

The perf profile had shown significant overhead in accel_move_string and zend_accel_hash_update. When Opcache processes a file, it must move strings from the compiler's temporary local memory into the persistent shared memory. If the interned string buffer is full, Opcache still attempts to process the file and manage the opcodes, repeatedly acquiring the zend_shared_alloc_lock mutex to update the hash table for the script itself, while simultaneously failing to intern the strings.

The cron script, running as a distinct CLI process, was constantly invalidating local string caches and forcing Opcache to attempt synchronization routines. Because the CLI process and the FPM workers were all interacting with a state where the string buffer was at absolute capacity, every file compilation or validation check forced a write-lock attempt on the shared memory segment, stalling all other processes waiting for read access.

The application code in the Arotech theme utilizes an extensive component library, translating thousands of translation keys, block pattern identifiers, and transient CSS class names into strings during runtime compilation. This specific operational characteristic rapidly populated the interned strings buffer beyond the default 8 MB allocation.

Memory Mapping Verification via GDB

To confirm the exact alignment of the shared memory segments and rule out a Linux kernel Virtual Memory Area (VMA) fragmentation issue, I attached the GNU Debugger (gdb) to a live, idle PHP-FPM worker process to inspect the memory mapping directly from the kernel's perspective.

First, I identified a target process identifier (PID).

pgrep -f "php-fpm: pool www" | head -n 1

The output yielded PID 104822. I attached gdb to this process.

gdb -p 104822

Once attached, the FPM worker was suspended. I instructed gdb to execute a shell command to read the /proc/[pid]/maps file, which details the memory-mapped files and shared memory segments for the process.

(gdb) shell grep opcache /proc/104822/maps

7f8a10000000-7f8a18000000 rw-s 00000000 00:01 1048576  /ZendUpcast.opcache (deleted)
7f8a20412000-7f8a20485000 r--p 00000000 103:02 8419202 /usr/lib/php/20220829/opcache.so
7f8a20485000-7f8a20510000 r-xp 00073000 103:02 8419202 /usr/lib/php/20220829/opcache.so
7f8a20510000-7f8a20545000 r--p 000fe000 103:02 8419202 /usr/lib/php/20220829/opcache.so
7f8a20545000-7f8a2054a000 r--p 00132000 103:02 8419202 /usr/lib/php/20220829/opcache.so
7f8a2054a000-7f8a2054e000 rw-p 00137000 103:02 8419202 /usr/lib/php/20220829/opcache.so

The first line 7f8a10000000-7f8a18000000 represented the actual Opcache shared memory segment. The rw-s flags indicate it is readable, writable, and shared. The (deleted) marker is standard behavior for anonymous shared memory mappings created via mmap; the file is unlinked from the filesystem immediately after creation so that it only exists in RAM, ensuring it is destroyed if the master process terminates.

I calculated the size of the mapping in bytes using the hexadecimal memory addresses.

(gdb) print/d 0x7f8a18000000 - 0x7f8a10000000

The output was 134217728. Converting this byte value: 134,217,728 / 1024 / 1024 = 128 MB. This confirmed the kernel had allocated exactly 128 MB for the Opcache segment, matching the default PHP configuration.

I detached gdb to allow the worker process to resume execution.

(gdb) detach
(gdb) quit

Reconfiguration of the Zend Engine

The resolution required altering the Opcache memory allocation limits to accommodate the string volume of the application stack. The configuration file 10-opcache.ini was located in /etc/php/8.2/fpm/conf.d/.

I opened the file for modification.

; configuration for php opcache module
; priority=10
zend_extension=opcache.so

opcache.enable=1
opcache.enable_cli=1

The first modification addressed the general memory consumption. While 128 MB was not fully exhausted (14.11 MB remained free), modern applications utilizing extensive vendor directories often exceed this baseline. I increased the total shared memory segment to 256 MB.

opcache.memory_consumption=256

The critical parameter was opcache.interned_strings_buffer. The default value is 8 MB. Based on the previous telemetry showing 142,051 strings consuming 7.99 MB, I quadrupled the allocation to 32 MB to provide sufficient headroom for future application deployments and string-heavy translation files.

opcache.interned_strings_buffer=32

The opcache.max_accelerated_files directive dictates the size of the internal hash table used to look up scripts. The value must be a prime number. If a non-prime number is configured, the Zend Engine rounds it up to the next available prime number in its predefined list. The previous setting allowed for 16,229 keys. I increased this to accommodate larger codebases without hash collisions.

opcache.max_accelerated_files=32531

I reviewed the opcache.validate_timestamps setting. When set to 1, Opcache checks the filesystem modification time (mtime) of the PHP script every opcache.revalidate_freq seconds. If the file has changed, the cache is invalidated. In a production environment where code is immutable between deployments, this stat operation introduces unnecessary I/O overhead and lock contention.

I disabled timestamp validation. This shifts the responsibility of clearing the Opcache entirely to the deployment pipeline, ensuring that the engine never attempts to stat the disk during standard HTTP request processing or cron executions.

opcache.validate_timestamps=0

Finally, I verified the Fast Shutdown mechanism. Fast shutdown allows the Zend Engine to bypass the standard, linear destructors for every object and variable in memory at the end of a request, instead relying on the Zend Memory Manager to free the entire block of request-bound memory at once. This significantly reduces process termination time. In PHP 8.2, this is enabled by default, but I explicitly defined it for configuration immutability.

opcache.fast_shutdown=1

I saved the configuration file.

Applying State and Verification

Because the Opcache shared memory segment is bound to the lifecycle of the FPM master process, modifying the configuration requires a complete service restart. Reloading the service (sending a SIGHUP signal) is insufficient, as the master process will not reallocate the underlying mmap segment.

I restarted the PHP-FPM daemon.

systemctl restart php8.2-fpm

Following the restart, I re-executed the CLI statistics script to verify the new memory topologies.

php /var/tmp/opcache_dump.php

{
    "memory": {
        "used": "42.10 MB",
        "free": "213.90 MB",
        "wasted": "0.00 MB",
        "current_wasted_percentage": "0%"
    },
    "interned_strings": {
        "buffer_size": "32 MB",
        "used_memory": "8.14 MB",
        "free_memory": "23.86 MB",
        "number_of_strings": 145112
    },
    "statistics": {
        "num_cached_scripts": 4210,
        "num_cached_keys": 6224,
        "max_cached_keys": 32531,
        "oom_restarts": 0,
        "hash_restarts": 0,
        "manual_restarts": 0,
        "misses": 4210,
        "hits": 140
    }
}

The new limits were correctly applied by the kernel. The interned_strings buffer now possessed 23.86 MB of free capacity. The total shared memory capacity reflected the 256 MB allocation.

During the subsequent 02:00 UTC operational window, the cron routine executed. Profiling the CPU instruction states with perf showed the zend_shared_alloc_lock mutex contention had entirely disappeared from the top execution overhead list. The P99 HTTP latency metric remained anchored at 48 milliseconds throughout the execution phase. The shared memory segmentation was stabilized.