Investigating Slab Allocator Fragmentation in Finazze MicroVMs

The environment is a production fleet utilizing hardened Alpine Linux 3.19 instances encapsulated within Firecracker microVMs. Each microVM is allocated 2 vCPUs and 1024MB of RAM. The workload is a deployment of the Finazze - Business and Finance WordPress Theme integrated into a decoupled architecture. The stack includes Nginx 1.26, PHP-FPM 8.3 with JIT enabled, and a remote MariaDB 11.2 instance. During the execution of the theme's asset pre-rendering pipeline—specifically when the financial dashboard generates localized SVG graphs and minified CSS modules—a subtle drift in TCP retransmission timeouts (RTO) was detected.

The networking layer for these microVMs is handled by a standard tap interface bridged to the host's physical NIC. The latency drift was not constant; it occurred exclusively during the theme's metadata-heavy filesystem operations. Monitoring indicated that RTO values occasionally exceeded the 200ms threshold, causing a momentary stall in ACK processing between the microVM and the remote database.

Observation: MicroVM Memory Pressure

Initial checks of CPU usage and I/O wait times showed no saturation. The CPU remained below 40% during the asset generation. However, the microVM's memory ballooning driver reported high reclamation activity. I shifted focus to the kernel's memory management, specifically the slab allocator. Unlike traditional virtual machines, Firecracker's restricted memory overhead makes kernel-level fragmentation visible much earlier.

A look at the VFS (Virtual File System) metrics indicated that the Finazze theme's directory structure is notably deep. The theme utilizes a modular approach, loading 42 unique directory components per request. In a free download WooCommerce Theme integration, this depth increases as child templates and asset manifests are queried across multiple search paths. Each lookup triggers a dentry allocation in the kernel.

Diagnostic Path: Slab Allocator Analysis

I utilized slabtop and queried /proc/slabinfo to audit the current state of kernel objects. The output revealed that dentry and inode_cache were consuming 342MB of the 1024MB available RAM. The dentry slab showed an efficiency of only 62%. This suggests that while millions of dentry objects were created, many were inactive yet held in the slab due to fragmentation within the SLUB allocator's pages.

slabinfo - version: 2.1
# name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab>
dentry            1124592       1813856    192       21           1
inode_cache       541202        782104     592       13           2

In the Linux kernel, the struct dentry serves as a glue between the inode and the filename. For every file check performed by PHP's file_exists() or is_readable() during the theme's initialization, the kernel must resolve the path. If the Finazze theme checks for wp-content/themes/finazze/assets/css/modules/dashboard/charts.css, the kernel performs a lookup for every directory segment. When the dentry slab is fragmented, these lookups involve traversing longer hash chains in the dentry cache, which consumes CPU cycles and increases memory bus contention.

Correlation: TCP RTO and VFS Pressure

The TCP stack in Linux depends on precise timing for RTT (Round Trip Time) estimation. When the kernel is busy reclaiming slab pages or traversing fragmented dentry hash tables, the interrupt handling for the virtio-net driver can be delayed. In a Firecracker environment, where the virtqueue processing is sensitive to vCPU scheduling, this delay causes the TCP stack to miscalculate the RTO.

The RTO drift was a direct symptom of the kernel's struggle to manage the VFS cache for the Finazze theme's asset lookups. As the slab allocator attempted to free up memory for the PHP-FPM worker processes, it triggered a series of "soft lockups" where the networking stack was temporarily starved of cycles. The RTO values would spike from a baseline of 1ms to over 200ms, triggering unnecessary retransmissions for database queries.

Deep Dive: Internal Dentry Structure

Each dentry object on this 64-bit Alpine kernel occupies 192 bytes. The structure includes: - d_flags: 4 bytes. - d_seq: 4 bytes. - d_hash: 8 bytes for the hlist node. - d_parent: 8 bytes (pointer to the parent dentry). - d_name: 8 bytes for the qstr structure. - d_inode: 8 bytes (pointer to the inode).

When the theme's modular CSS generator runs, it frequently performs stat() calls on hundreds of files to verify cache integrity. This behavior, combined with the free download WooCommerce Theme logic that searches through localized .mo files, fills the dentry slab rapidly.

I examined the SLUB allocator's per-CPU cache. Each vCPU maintains a kmem_cache_cpu structure which stores a pointer to a "partial" slab. When the Finazze theme's requests are distributed across both vCPUs, the partial slabs become fragmented. The allocator is forced to request more pages from the buddy allocator, but because the microVM memory is limited, it fails, leading to the churn observed in /proc/slabinfo.

Analyzing the SLUB Allocator Mechanics

The SLUB allocator, which is the default in modern Linux kernels, aims for better scalability by reducing lock contention. However, in low-memory microVMs, its "best-fit" logic can lead to memory waste. Each slab page (usually 4KB or 8KB) contains several objects. If a single dentry in a slab page remains active, the entire page cannot be reclaimed.

The Finazze theme's pre-rendering pipeline creates many short-lived dentries. If one dentry per page is pinned by an open file handle or a long-running PHP-FPM process, the memory footprint remains high while the active_objs count remains low. I monitored the num_slabs vs active_slabs metrics. The discrepancy was 38%, which represents the memory lost to internal fragmentation within the slab pages.

Filesystem Metadata and VirtIO-Block Latency

The filesystem used is XFS. I checked the xfs_inode slab. XFS uses its own caching mechanism which interacts with the VFS. The Finazze theme's high frequency of unlink() and rename() calls during asset minification causes XFS to frequently update its internal b-trees. These updates require dentry invalidation.

In Firecracker, the block device is emulated via VirtIO-block. When the VFS needs to fetch an inode that has been evicted from the slab, it must perform a VirtIO request. The latency of this request adds to the overall stall. If the kernel is already under slab pressure, the thread handling the I/O completion can be delayed. This creates a feedback loop: high VFS pressure leads to slab churn, which leads to more block I/O, which delays the TCP stack, resulting in the RTO drift.

The Role of PHP's Realpath Cache

PHP attempts to mitigate VFS lookups using its internal realpath_cache. For the Finazze theme, I increased the realpath_cache_size to 16M. While this reduced the number of stat() calls reaching the kernel, it did not eliminate the slab pressure during the initial boot and cache-clear phases.

The realpath cache stores the resolved paths in the PHP worker's private heap. However, the worker still must verify the file's existence occasionally. In a microVM with only 1024MB of RAM, allocating 16MB per worker (with 10 workers) consumes 160MB. This further reduces the memory available for the kernel's slab allocator, exacerbating the fragmentation.

Networking Stack Adjustments

To handle the RTO drift, I looked at the tcp_retries2 and tcp_reordering parameters. On a reliable bridge connection, these are rarely the problem. However, the microVM bridge can experience micro-congestion. I increased the tcp_reordering from 3 to 10 to reduce the sensitivity to out-of-order packets caused by the CPU scheduling delays.

I also checked the TCP_METRICS cache. The kernel stores RTT and RTO values for remote hosts.

ip tcp_metrics show

The metrics for the MariaDB host showed an RTT variance of 145ms. This confirmed that the kernel was correctly identifying the latency but was unable to respond due to the VFS stalls. The solution was not in the networking stack but in how the kernel prioritized metadata caching.

Tuning VFS Cache Pressure

The kernel parameter vm.vfs_cache_pressure determines how aggressively the kernel reclaims dentries and inodes relative to page cache. The default value is 100. For the Finazze theme's modular structure, I found that the default value was too high. The kernel was reclaiming dentries too early, leading to the churn and the subsequent TCP delays.

I adjusted the value to 50. This instructs the kernel to prioritize the retention of dentries and inodes. While this increases the memory footprint of the slab, it reduces the CPU cycles spent on re-traversing the filesystem. In the context of a microVM, this is a trade-off: more memory for metadata means less memory for PHP, but it results in a more stable TCP RTO.

Memory Allocation in Musl Libc

Alpine uses musl instead of glibc. Musl's allocator is designed for simplicity and lower overhead. However, it does not support "arenas" in the same way glibc does. For a PHP-FPM workload, this means each worker's memory allocation is more direct. When the kernel experiences slab pressure, the musl allocator's requests for new pages can be delayed by the kernel's reclaim logic.

I monitored the MALLOC_ARENA_MAX equivalent behavior in musl. While it doesn't exist, the fragmentation within musl's heap was minimal compared to the kernel's slab fragmentation. This confirmed that the root of the RTO drift was exclusively in the kernel's handling of VFS metadata for the Finazze theme.

virtio-net Ring Buffer Configuration

I audited the virtio-net ring buffer sizes. In Firecracker, the default is often 256 entries. When the Finazze theme's asset pipeline generates thousands of small file requests, the network traffic (even if just database queries) can saturate these small buffers if the vCPU is occupied by slab reclamation.

I increased the queue size to 1024. This provided a larger buffer for incoming ACKs, allowing the networking stack to survive the momentary CPU stalls caused by the dentry hash table traversal. This adjustment, combined with the vfs_cache_pressure change, stabilized the RTO values.

Cache Invalidation and Slab Recovery

When the Finazze theme's cache is purged, the kernel must invalidate millions of dentry objects. This process is not instantaneous. The d_invalidate function must lock the dentry hash table. During this phase, any networking operation that requires a path resolution—such as reading a configuration file or a database certificate—will be blocked.

I found that the theme's cache purging was done via rm -rf. A more efficient approach for this environment was to move the cache directory to a tmpfs mount and simply unmount and remount it. This offloads the dentry management to a RAM-based filesystem which has significantly lower overhead for invalidation.

tmpfs Offloading for Metadata Efficiency

By moving the wp-content/cache and wp-content/themes/finazze/assets/generated directories to tmpfs, the dentry allocations for these files no longer persisted in the main filesystem's metadata slabs. Tmpfs handles dentries and inodes differently; it stores them in the page cache directly.

This reduced the dentry slab size from 342MB to 110MB. The SLUB allocator efficiency improved to 94%. More importantly, the TCP RTO drift disappeared. The kernel no longer had to perform XFS b-tree updates for the temporary files generated by the Finazze theme.

Analyzing the CPU Scheduler Interaction

Firecracker uses the host's KVM and the standard Linux scheduler (CFS or EEVDF). I noticed that the microVM vCPUs were being descheduled during high I/O wait. When the Finazze theme requested a file not in the dentry cache, the vCPU would enter an idle state waiting for the virtio-block response.

During this idle state, the host scheduler might move the vCPU to a different physical core. This migration introduces cache misses in the CPU's L1/L2 caches, further slowing down the dentry lookup once the I/O completes. By pinning the microVM vCPUs to specific host cores, I reduced the scheduling jitter.

Final Verification of the TCP Stack

After implementing the vfs_cache_pressure tuning, increasing the virtio-net ring buffers, and offloading temporary assets to tmpfs, I re-ran the pre-rendering pipeline. The RTO values for the MariaDB host were monitored via ss -i.

ss -i state established dst [DATABASE_IP]

The rto was reported at a consistent 1.2ms. The unacked count was zero. The Finazze dashboard generated its assets in 12.4 seconds, down from 18.2 seconds. The kernel slab allocator reported a stable footprint with no sign of the churn that had characterized the previous state.

Filesystem Mount Options and Atimes

To further optimize the VFS layer, I reviewed the mount options for the root filesystem. The use of relatime is standard, but for the Finazze theme, it was unnecessary. Every file access was resulting in a metadata write to update the access time. In a microVM, this is extra VirtIO overhead.

I switched to noatime,nodiratime. This eliminated the access time updates, reducing the number of b-tree updates in XFS. This significantly reduced the work required by the dentry invalidation logic and freed up more CPU cycles for the networking stack to handle TCP ACKs.

I/O Scheduler and VirtIO Tuning

The host I/O scheduler also plays a role. Since the host uses NVMe drives, I ensured the scheduler was set to none. In the microVM, the VirtIO-block driver does not use a scheduler by default. This is optimal as it allows the host to manage the I/O priority.

The Finazze theme's performance in this minimalist environment is a testament to the fact that even high-quality themes can be bottlenecked by infrastructure-level metadata handling. By focusing on the slab allocator and the VFS layer, rather than the PHP code itself, the throughput of the entire financial application was improved.

The free download WooCommerce Theme integration now functions with a TTFB (Time To First Byte) of 42ms, down from 88ms. The microVM's memory utilization is predictable, and the TCP RTO drift has been fully mitigated.

The following configuration ensures the kernel maintains a lean metadata footprint while avoiding TCP stalls during heavy asset generation.

# sysctl.conf adjustments for Finazze MicroVM environments
vm.vfs_cache_pressure = 50
net.ipv4.tcp_reordering = 10
net.core.netdev_max_backlog = 5000
net.ipv4.tcp_slow_start_after_idle = 0

# PHP-FPM optimization
php_admin_value[realpath_cache_size] = 16M
php_admin_value[realpath_cache_ttl] = 600

Avoid relying on default slab reclamation policies in low-memory microVM environments. Offload high-churn directory structures to tmpfs to bypass VFS metadata slabs entirely.