Debugging PHP-FPM Memory Leakage via GDB and Zend Internals

The environment is a cluster of Debian 12 nodes running PHP 8.2.14-FPM behind Nginx 1.24. The application layer is built on the Picart – Fashion WooCommerce WordPress Theme, which handles high-resolution fashion product archives. During routine monitoring of the PHP-FPM pool, I observed a consistent drift in the Resident Set Size (RSS) of the worker processes. A worker would start at 34MB and climb to 82MB after exactly 150 requests to the product archive page, even when the memory_limit was strictly capped.

Observation: RSS Drift and Memory Ceiling

The drift was not a typical leak where memory grows indefinitely until an OOM event. Instead, the RSS would plateau at a specific point, suggesting fragmentation within the Zend Memory Manager (ZMM) rather than a failure to free memory at the end of a request. The Picart theme utilizes an extensive set of associative arrays to manage fashion product attributes such as fabric weight, color swatches, and size availability. These arrays are reconstructed on every request through the woocommerce_get_product_attributes hook.

I initiated a deeper investigation into the ZMM's behavior. The Zend Engine uses its own heap manager to reduce the overhead of system-level malloc() and free() calls. The heap is divided into chunks, pages, and slots. When a script requests memory, ZMM attempts to fit the allocation into a pre-defined slot size (e.g., 8, 16, 32, up to 3072 bytes). For larger allocations, it uses pages.

Diagnostic Path: GDB and Zend Heap Inspection

I avoided standard profiling tools to minimize interference. I attached gdb to a running PHP-FPM worker that had processed 100 requests.

gdb -p <pid>

By loading the PHP source symbols and using the zbacktrace command, I identified that the memory was being held in the zend_hash_real_init and zend_hash_add_new functions. The specific culprit was the way the Picart theme merges attribute arrays. Every fashion attribute in the Picart theme creates a unique zval for the swatch metadata. These zvals are stored in a hash table.

When analyzing a Download WooCommerce Theme repository for comparison, it became evident that many themes suffer from "Array Churn." In the case of Picart, the theme performs repeated array_merge operations within a foreach loop to calculate the "Available Styles" sidebar. Each array_merge creates a new array, copies the data, and destroys the old one.

In the Zend Memory Manager, this creates a high volume of small allocations. If the size of the array grows incrementally, it moves from a 128-byte slot to a 256-byte slot, then to a 512-byte slot. When the old array is freed, the ZMM marks the slot as free but does not return it to the OS. If subsequent requests cannot perfectly fill these freed slots—perhaps because they require 64-byte or 1024-byte slots—the heap becomes fragmented. The RSS of the process grows to accommodate new chunks, while the internal heap is full of holes.

Zend Memory Manager Internals: The _zend_mm_heap Structure

To confirm this, I examined the _zend_mm_heap structure using GDB:

(gdb) print *php_fpm_worker_heap

The output showed that the free_slot list for the 512-byte bin was unusually long, while the chunks_count was increasing. This is the definition of heap fragmentation. The _zend_mm_alloc_int function in zend_alloc.c was failing to find a contiguous block in existing pages, forcing a new mmap call.

The Picart theme's attribute logic was generating a new hash table for every product in the loop. For a page with 40 products, the ZMM performs thousands of allocations. Because the number of attributes per product varies—some have 3 colors, some have 15—the memory manager is forced to juggle various slot sizes.

Bit-Level Allocation and Alignment

The ZMM uses a bitmask to track free pages in a chunk. A chunk is typically 2MB. Each chunk contains 512 pages of 4KB each. The first page is used for chunk metadata. When the Picart theme allocates a large associative array for the "Fabric Details" section, it might require three contiguous pages. If the heap is fragmented with single-page "holes," the allocator must request a new chunk from the kernel via brk() or mmap().

I looked at the ZEND_MM_ALIGNED_SIZE macro. PHP rounds up every allocation to the nearest 8-byte boundary (on 64-bit systems). However, for the fashion attributes in Picart, many strings are short (e.g., "Red", "Blue", "Silk"). These small strings are interned if possible, but the dynamic arrays holding them are not.

Kernel Interaction: Transparent Hugepages (THP)

The interaction with the Linux kernel's memory management also played a role. On this Debian node, transparent_hugepages was set to always. While THP can improve performance by reducing TLB misses, it exacerbates the RSS drift in fragmented heaps. When ZMM requests a new 2MB chunk, the kernel provides a 2MB huge page. Even if ZMM only uses 10KB of that chunk, the entire 2MB is accounted for in the process's RSS.

I changed the THP setting to madvise to allow the ZMM to explicitly request huge pages only when necessary, which it rarely does for small-slot allocations.

echo madvise > /sys/kernel/mm/transparent_hugepage/enabled

OPcache Interned Strings and Hash Tables

The Picart theme's localization files also contributed to the internal memory overhead. Each time a fashion attribute is translated, a new string is generated. If opcache.interned_strings_buffer is full, these strings are allocated on the request heap rather than in the shared memory segment.

I monitored the opcache_get_status() output: - interned_strings_usage.used_memory: 7.9MB - interned_strings_usage.buffer_size: 8MB

The buffer was nearly exhausted. Once full, every subsequent translated attribute name in the Picart theme was being allocated on the per-process heap, then freed, then re-allocated, increasing the fragmentation of the Zend slots.

Analyzing the Patch for Picart Theme

To address the root cause, I modified the theme's attribute collector. Instead of using array_merge in a loop, I shifted to using the [] operator to build a multi-dimensional array and then flattened it once at the end. This reduces the number of intermediate hash table allocations from N to 1.

Original Picart Logic (simplified):

$all_attributes = [];
foreach ($products as $product) {
    $all_attributes = array_merge($all_attributes, $product->get_attributes());
}

Modified Logic:

$buffer = [];
foreach ($products as $product) {
    $buffer[] = $product->get_attributes();
}
$all_attributes = array_merge(...$buffer);

This simple change reduced the number of ZMM large slot allocations by 90% per request. The fragmentation decreased, and the RSS stabilized after the initial 20 requests.

Tuning the Allocator and PHP-FPM Lifecycle

Even with the code patch, the nature of the Picart theme's fashion grids means some fragmentation is inevitable. I adjusted the PHP-FPM pm.max_requests to force a process restart before the RSS reaches the physical RAM limits of the VM.

I also tuned the ZMM environmental variables. By setting USE_ZEND_ALLOC=0, one can bypass ZMM and use the system malloc(), which is useful for debugging with tools like Valgrind, but for production, ZMM is superior if the code is clean. Instead, I stayed with ZMM but optimized the opcache settings to prevent string overflow.

Final Configuration and Sysctl Adjustments

The final adjustment involved the vm.overcommit_memory setting. In a fragmented heap environment, the kernel might refuse allocations even if free RAM appears available on the surface. Setting this to 1 (Always overcommit) allows the ZMM to claim virtual address space for its chunks without being blocked by the kernel's conservative heuristic.

# /etc/sysctl.conf
vm.overcommit_memory = 1
vm.swappiness = 5

The PHP-FPM pool configuration for the Picart theme:

[picart_pool]
user = www-data
group = www-data
listen = /run/php/php8.2-fpm-picart.sock
pm = static
pm.max_children = 50
pm.max_requests = 250

; Increase interned strings to prevent heap overflow
php_admin_value[opcache.interned_strings_buffer] = 32
php_admin_value[opcache.memory_consumption] = 256
php_admin_value[memory_limit] = 128M

The pm.max_requests = 250 ensures that even if minor fragmentation occurs during complex fashion attribute rendering, the process will be recycled before the RSS drifts beyond 60MB.

Monitor your Zend heap via gdb if you see RSS drift. High free_slot counts in the mid-range bins (256-1024 bytes) usually indicate inefficient array merging in the theme layer. Stop using array_merge in loops. Use the spread operator or temporary buffers. Keep your process lifecycle short enough to mask fragmentation but long enough to benefit from JIT and OPcache.