Debugging systemd cgroup v2 Memory Pressure Stalls (PSI), Anonymous Swapping & Direct Reclaim Latency under PHP-FPM and MariaDB Burst Workloads

Under high-concurrency production workloads—such as high-traffic WooCommerce checkout flash sales, multi-tenant cPanel environments, or API microservices handling concurrent database writes—Linux servers often experience sudden, crippling latency spikes.

System administrators and site reliability engineers frequently encounter a perplexing phenomenon: while overall CPU usage appears moderate (30% to 50%) and physical RAM seems to show available headroom, HTTP Time-To-First-Byte (TTFB) degrades from 45ms to over 8,000ms. In severe cases, database connections stall, php-fpm pools queue incoming FastCGI requests until Nginx returns 504 Gateway Time-out, or database processes terminate abruptly without an entry in /var/log/messages from the legacy kernel Out-Of-Memory (OOM) killer.

[Tue Sep 08 05:42:19.821049 2026] [proxy_fcgi:error] [pid 184209:tid 184291] [client 198.51.100.42:54210] 
AH01075: Error dispatching request to : (polling) - idle timeout (60s) reached, referer: https://example.com/checkout/
systemd[1]: system.slice/mariadb.service: A process of this unit has been killed by the OOM killer.
systemd-oomd[742]: Killed /system.slice/mariadb.service due to memory pressure (some avg10=64.82% > 50.00%)

The underlying culprit is typically not pure physical memory exhaustion, but Kernel Direct Reclaim Lockups, Anonymous Page Thrashing, and uncalibrated cgroup v2 Pressure Stall Information (PSI) thresholds enforced by userspace daemons like systemd-oomd.

This guide provides an exhaustive architectural deep dive into the Linux 6.x memory controller under cgroup v2. We explore the physics of page reclaim watermarks, analyze PSI telemetry, trace direct reclaim latency using eBPF (bpftrace), and implement production-hardened systemd slice reservations for High-Performance Linux VPS and Dedicated Enterprise Clusters.

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1. Linux Memory Management Architecture under cgroup v2

In Linux cgroup v1, memory controllers operated with isolated subsystems (memory, blkio, cpu) that failed to correlate page cache writeback with block I/O attribution, leading to priority inversions and uncoordinated memory reclaim.

The unified hierarchy of cgroup v2 fixes these flaws by implementing integrated memory and I/O tracking, multi-tiered memory boundaries (min, low, high, max), and Pressure Stall Information (PSI).

+-------------------------------------------------------------------------------+
|                             Linux Kernel 6.x VFS / MM                         |
+-------------------------------------------------------------------------------+
                                       |
                   +-------------------+-------------------+
                   |                                       |
                   v                                       v
        +----------------------+               +----------------------+
        |   Anonymous Memory   |               |   File-Backed Cache  |
        |  (PHP Heap, MariaDB  |               | (Inodes, Dentries,   |
        |   InnoDB Buffer Pool)|               |  Web Assets, Binaries|
        +----------+-----------+               +-----------+----------+
                   |                                       |
                   | (vm.swappiness, vm.vfs_cache_pressure)|
                   v                                       v
+-------------------------------------------------------------------------------+
|                       Kernel Watermark Evaluation Engine                      |
|                                                                               |
|   High Watermark  -------> [ Normal Operation / No Pressure ]                 |
|   Low Watermark   -------> [ kswapd Asynchronous Background Reclaim ]         |
|   Min Watermark   -------> [ DIRECT RECLAIM: Synchronous Worker Stall! ]      |
+-------------------------------------------------------------------------------+
                                       |
        +------------------------------+------------------------------+
        |                                                             |
        v                                                             v
+-------------------------------+             +-------------------------------+
|  cgroup v2: php-fpm.service   |             |  cgroup v2: mariadb.service   |
|  - memory.min (Guaranteed)    |             |  - memory.min (Guaranteed)    |
|  - memory.low (Soft Prot)     |             |  - memory.low (Soft Prot)     |
|  - memory.high (Throttled)    |             |  - memory.high (Throttled)    |
|  - memory.max (Hard Limit)    |             |  - memory.max (Hard Limit)    |
|  - memory.pressure (PSI)      |             |  - memory.pressure (PSI)      |
+-------------------------------+             +-------------------------------+

Memory Classification: Anonymous vs. File-Backed Pages

To diagnose memory stalls, we must differentiate how the kernel handles the two primary memory types:

1. File-Backed Pages (file): Mapped to files on disk (e.g., PHP scripts, Nginx static assets, binary executables). When memory pressure rises, clean file pages can be dropped instantly without writing to disk. Dirty file pages require flushing via the writeback engine (flusher threads).
2. Anonymous Memory (anon): Dynamically allocated process memory not backed by a filesystem file (e.g., PHP-FPM variable heaps, OPCache internal arrays, MariaDB innodb_buffer_pool, sorting buffers). Anonymous pages cannot simply be discarded; to reclaim an anonymous page, the kernel must write it to swap (or compress it into zswap/zRAM).

When PHP-FPM processes rapidly spawn and burst their memory allocations during traffic spikes, the kernel must free pages immediately. If clean page caches are depleted and anonymous memory dominates, the kernel enters expensive swap and compaction cycles.

---

2. The Physics of Pressure Stall Information (PSI)

Linux Pressure Stall Information (PSI) quantifies the productivity loss caused by resource shortages (CPU, Memory, I/O). Unlike simple utilization metrics (which only measure capacity consumed), PSI measures execution latency lost due to starvation.

Memory PSI is exposed in /proc/pressure/memory globally, and at /sys/fs/cgroup/<slice>/<service>/memory.pressure per cgroup:

some avg10=12.45 avg60=8.20 avg300=3.10 total=48291044
full avg10=4.12 avg60=1.85 avg300=0.45 total=12948210

Deconstructing some vs. full Metrics

• some: Represents the percentage of wall-clock time in which at least one non-idle thread was stalled waiting for memory resources (e.g., delayed in direct reclaim, waiting for page swap-in, or blocked on memory compaction).
• full: Represents the percentage of time in which all runnable threads within the cgroup (or system) were simultaneously blocked waiting for memory. During a full memory stall, CPU capacity is completely wasted because no thread can make forward progress.

$$\text{Stall Percentage} = \left( \frac{\Delta \text{Stall Time}}{\Delta \text{Wall Time}} \right) \times 100$$

A sustained some avg10 > 25.0 indicates severe memory contention where web workers spend a quarter of their time waiting on page allocations. A full avg10 > 5.0 will result in dropped TCP SYN queues and 504 gateway timeouts.

---

3. Direct Reclaim vs. kswapd: Why Concurrency Bursts Lock the Kernel

The Linux memory allocator uses three internal page watermarks per NUMA zone: High, Low, and Min.

       Zone Memory Watermarks
+------------------------------------+  Total Zone Memory
|                                    |
|         Normal Free Memory         |
|                                    |
+------------------------------------+  <- High Watermark
|   kswapd Wakes Up in Background    |
+------------------------------------+  <- Low Watermark
|   DIRECT RECLAIM (Synchronous)     |
+------------------------------------+  <- Min Watermark (vm.min_free_kbytes)
|   OOM Killer Invocation Zone       |
+------------------------------------+  0 Free Pages

1. Above High Watermark: Allocations succeed instantly from the buddy allocator freelist with zero latency.
2. Between Low and High Watermark: As free pages drop below Low, the kernel wakes the asynchronous background reclaim daemon (kswapd). kswapd scans the inactive page list and frees pages asynchronously without blocking application threads.
3. Below Min Watermark: If memory allocation velocity exceeds kswapd throughput (e.g., 60 concurrent PHP-FPM workers allocating 32MB each within 200ms), free pages drop below Min.

When this happens, the kernel forces the calling process into Synchronous Direct Reclaim:

php-fpm worker process (PID 14208)
  -> malloc() / do_anonymous_page()
    -> alloc_pages_slowpath()
      -> __perform_reclaim()
        -> try_to_free_pages()
          -> do_try_to_free_pages()
            -> shrink_node()
              -> shrink_active_list()
              -> pageout() -> [Blocks on Block Layer / Swap I/O]

In direct reclaim, every PHP-FPM worker thread halts execution of user code, switching to kernel space to scan page lists, lock page tables, write dirty anonymous pages to swap, or wait on synchronous page compaction.

If 40 workers enter direct reclaim simultaneously, they contend for the same memory zone locks (lru_lock), causing CPU %sys time to jump to 70%+ while execution throughput drops to zero.

---

4. Deep Diagnostic Telemetry & Kernel Tracing

Let us execute real-time diagnostics to identify memory pressure stalls, trace direct reclaim duration, and verify whether systemd-oomd is terminating active processes.

Step 1: Inspect System and cgroup PSI

Check global memory pressure:

cat /proc/pressure/memory

Output:

some avg10=42.18 avg60=28.94 avg300=14.12 total=184920482
full avg10=18.45 avg60=11.20 avg300=4.15 total=54910214

Check cgroup v2 memory pressure specifically for PHP-FPM and MariaDB:

cat /sys/fs/cgroup/system.slice/php-fpm.service/memory.pressure
cat /sys/fs/cgroup/system.slice/mariadb.service/memory.pressure

Inspect active memory counters and events inside the cgroup:

cat /sys/fs/cgroup/system.slice/php-fpm.service/memory.stat | grep -E "anon|file|sock|shmem|pgfault|pgmajfault"
cat /sys/fs/cgroup/system.slice/php-fpm.service/memory.events

Sample output:

low 0
high 14209
max 84
oom 12
oom_kill 12
oom_group_kill 0

Diagnostic Insight: An incrementing high counter indicates that processes in this cgroup crossed memory.high and were forcefully throttled by the kernel (induced sleep delays). An incrementing oom_kill indicates processes were terminated by the cgroup memory controller.

Step 2: Correlating Virtual Memory Scan and Reclaim Activity with sar & vmstat

Monitor real-time page scanning rates:

vmstat -S M 1 10

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
18  6   1420    210     45   1820  145  420  1820  2450 8420 19450 32 48  4 16  0
24  8   1480    185     42   1780  210  680  2400  3100 9850 24100 28 54  2 16  0

Notice b (blocked processes) climbing alongside high so (swap out), high sy (kernel system time), and low id (idle time).

Next, run sar -B 1 5 to inspect direct reclaim vs kswapd activity:

sar -B 1 5

Linux 6.6.0-enterprise-amd64 (node01.nextgen.pk) 	09/08/2026 	_x86_64_	(16 CPU)

05:50:01 AM  pgpgin/s pgpgout/s   fault/s  majflt/s  pgfree/s pgscank/s pgscand/s pgsteal/s %vmeff
05:50:02 AM  4820.00  9410.00  18450.00    248.00  28410.00   1200.00  34120.00  32100.00  90.88
05:50:03 AM  6120.00 12840.00  22100.00    312.00  31200.00    850.00  42800.00  39500.00  89.70

• pgscank/s (kswapd page scan): Asynchronous background scanning. Low numbers mean kswapd is falling behind.
• pgscand/s (direct page scan): Synchronous direct reclaim. Numbers above 1,000/s indicate severe thread stalling.
• majflt/s (major page faults): Processes blocking to read pages from disk or swap.

Step 3: Kernel-Level Direct Reclaim Tracing with eBPF (bpftrace)

To observe the exact latency imposed on web worker threads by kernel direct reclaim, deploy the following eBPF one-liner:

bpftrace -e '
kprobe:try_to_free_pages {
    @start[tid] = nsecs;
}

kretprobe:try_to_free_pages /@start[tid]/ {
    $duration_us = (nsecs - @start[tid]) / 1000;
    @reclaim_latency_us = hist($duration_us);
    @reclaim_by_comm[comm] = sum($duration_us);
    delete(@start[tid]);
}

interval:s:10 {
    print(@reclaim_latency_us);
    print(@reclaim_by_comm);
    clear(@reclaim_latency_us);
    clear(@reclaim_by_comm);
}
'

Output during a traffic burst:

@reclaim_latency_us: 
[256, 512)           142 |@@@@                                            |
[512, 1K)            850 |@@@@@@@@@@@@@@@@@@@@@@@@                        |
[1K, 2K)            1420 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@        |
[2K, 4K)            1890 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[4K, 8K)             620 |@@@@@@@@@@@@@@@@                                |
[8K, 16K)            180 |@@@@                                            |
[16K, 32K)            45 |@                                               |
[32K, 64K)            12 |                                                |

@reclaim_by_comm[php-fpm]: 14820914 us
@reclaim_by_comm[mariadbd]: 4210940 us
@reclaim_by_comm[nginx]: 124010 us

Analysis: Over a 10-second sampling window, php-fpm workers spent a cumulative 14.82 seconds suspended in try_to_free_pages. Individual requests were delayed by up to 32 milliseconds per allocation, destroying PHP application throughput.

---

5. Architectural Root Causes & Misconfigurations

Three interrelated misconfigurations cause these failure cascades:

1. The Dynamic MariaDB + PHP-FPM Resource Overcommit Trap

When configuring MariaDB and PHP-FPM on the same server, administrators frequently budget memory using static peak numbers:

• Physical RAM: 32 GB
• MariaDB innodb_buffer_pool_size: 20 GB
• PHP-FPM pm = dynamic, pm.max_children = 120

Under heavy load: $$\text{Max PHP Memory} = 120 \times 128,\text{MB} = 15.36,\text{GB}$$ $$\text{Total Peak Demand} = 20,\text{GB (MariaDB)} + 15.36,\text{GB (PHP)} + 1.5,\text{GB (System/Nginx)} = 36.86,\text{GB}$$

When concurrency surges, total memory demand exceeds 32 GB. Because neither service has cgroup v2 memory protections, the kernel reclaims file caches to 0. As soon as file cache cannot satisfy the demand, the kernel forces PHP-FPM anonymous memory into swap.

When MariaDB executes queries needing temporary tables, MariaDB and PHP-FPM aggressively compete for memory allocations, triggering direct reclaim and cascading timeouts.

2. systemd-oomd Killing Services on Temporary Pressure

Modern distributions (Ubuntu 22.04+, Debian 12+, RHEL 9+, Fedora) enable systemd-oomd by default. systemd-oomd polls cgroup memory.pressure every second. If some avg10 exceeds the default threshold (often 50% for more than 20 seconds), systemd-oomd chooses the cgroup with the largest swap or memory footprint and kills all processes in that unit.

MariaDB, holding 20 GB of InnoDB buffer pool, is frequently selected as the “fattest” target and killed instantly, causing severe database corruption risks and complete downtime.

3. Sub-Optimal Linux VM Watermark Gap (vm.watermark_scale_factor)

By default, the Linux kernel sets /proc/sys/vm/watermark_scale_factor to 10 (representing 0.1% of memory zone size). On a 32 GB server, the gap between the Low and Min watermark is only ~32 MB.

When 50 PHP-FPM workers suddenly wake up and allocate 200 MB of heap memory, they blow through that 32 MB buffer in milliseconds. kswapd does not have sufficient time to free pages asynchronously, plunging the system immediately into direct reclaim.

---

6. Step-by-Step Resolution & Production Engineering Hardening

To permanently resolve memory pressure stalls and prevent rogue OOM terminations, implement this comprehensive, four-layer configuration framework.

+-------------------------------------------------------------------------------+
|                       Production Hardening Framework                          |
+-------------------------------------------------------------------------------+
| 1. Systemd cgroup v2 Isolation (MemoryMin, MemoryLow, MemoryHigh, OOMScore)   |
| 2. Kernel VM Watermark & Swappiness Tuning (sysctl.d)                         |
| 3. Fast In-Memory Swap Architecture (ZRAM / Zswap with ZSTD)                  |
| 4. Workload-Specific Heap Calibration (MariaDB InnoDB & PHP-FPM Pools)        |
+-------------------------------------------------------------------------------+

---

Step 1: Configure systemd cgroup v2 Resource Isolation

We will configure systemd drop-in overrides for mariadb.service and php-fpm.service to establish explicit memory boundaries and prevent systemd-oomd from targeting the database.

A. MariaDB Service Protection Drop-in

Create the override directory and file:

mkdir -p /etc/systemd/system/mariadb.service.d/
cat <<'EOF' > /etc/systemd/system/mariadb.service.d/10-cgroup-memory.conf
[Service]
# Guarantee MariaDB memory is never reclaimed under pressure
MemoryMin=12G
MemoryLow=16G

# Soft limit to throttle background jobs, hard limit to prevent kernel panic
MemoryHigh=24G
MemoryMax=26G

# Disallow swapping MariaDB buffer pool to disk
MemorySwapMax=0

# Protect MariaDB from systemd-oomd and kernel OOM Killer
ManagedOOMMemoryPressure=ignore
ManagedOOMPreference=avoid
OOMScoreAdjust=-800

# High-concurrency task allowance
TasksMax=8192
EOF

B. PHP-FPM Service Resource Control Drop-in

For PHP-FPM (replace php8.3-fpm or ea-php83-php-fpm with your active PHP version):

mkdir -p /etc/systemd/system/php8.3-fpm.service.d/
cat <<'EOF' > /etc/systemd/system/php8.3-fpm.service.d/10-cgroup-memory.conf
[Service]
# Provide base guarantee for running workers
MemoryLow=4G

# Soft limit where kernel begins gentle throttling instead of instant kill
MemoryHigh=10G

# Absolute ceiling to prevent rogue PHP memory leaks from crashing the node
MemoryMax=12G

# Allow limited zswap/swap to absorb transient burst spikes
MemorySwapMax=2G

# If PHP-FPM exceeds memory bounds, sacrifice worker processes before the DB
ManagedOOMMemoryPressure=kill
ManagedOOMMemoryPressureLimit=65%
ManagedOOMPreference=omit
OOMScoreAdjust=200

# CPU scheduling priority
CPUWeight=100
IOWeight=100
EOF

Apply the changes:

systemctl daemon-reload
systemctl restart mariadb php8.3-fpm

Verify the cgroup attributes applied:

cat /sys/fs/cgroup/system.slice/mariadb.service/memory.min
cat /sys/fs/cgroup/system.slice/mariadb.service/memory.low
cat /sys/fs/cgroup/system.slice/php8.3-fpm.service/memory.high

---

Step 2: Linux Virtual Memory Subsystem Optimization (sysctl)

Tune the kernel memory subsystem to widen watermark thresholds, preventing sudden drops into direct reclaim.

Create /etc/sysctl.d/99-memory-pressure-tuning.conf:

# /etc/sysctl.d/99-memory-pressure-tuning.conf

# 1. Widen watermark gap (Default 10 = 0.1%, Set to 200 = 2% of RAM)
# On 32GB RAM, gives kswapd a 640MB buffer to reclaim pages BEFORE direct reclaim stalls
vm.watermark_scale_factor = 200

# 2. Reserve emergency memory for atomic kernel allocations and NIC ring buffers
vm.min_free_kbytes = 262144

# 3. Modern swappiness setting with cgroup v2
# With Zswap/ZRAM enabled, swappiness=60 allows kernel to balance anon vs file reclaim effectively
vm.swappiness = 60

# 4. Prevent premature directory/inode cache eviction
vm.vfs_cache_pressure = 50

# 5. Dirty page writeback tuning to prevent block I/O writeback stalls
# Background flusher starts at 5% dirty memory
vm.dirty_background_ratio = 5
# Applications block on synchronous writeback only if dirty memory hits 15%
vm.dirty_ratio = 15
vm.dirty_expire_centisecs = 1500
vm.dirty_writeback_centisecs = 500

# 6. Memory overcommit policy (0 = Heuristic overcommit)
vm.overcommit_memory = 0
vm.overcommit_ratio = 50

# 7. Compaction proactivity
vm.compaction_proactiveness = 30

Load the sysctl configuration:

sysctl -p /etc/sysctl.d/99-memory-pressure-tuning.conf

---

Step 3: Fast In-Memory Swap Architecture (Zswap / ZRAM)

Disk-backed swap on standard NVMe drives introduces microsecond-to-millisecond I/O wait latency during page-out/page-in operations. By configuring Zswap with the high-throughput zstd or lz4 compression algorithm, anonymous pages are compressed in-RAM in nanoseconds.

Check if zswap is active:

cat /sys/module/zswap/parameters/enabled

If N, configure zswap persistently via sysfs or kernel boot parameters:

# Set high-speed compression algorithm
echo zstd > /sys/module/zswap/parameters/compressor

# Set 3-to-1 memory allocator
echo z3fold > /sys/module/zswap/parameters/zpool

# Allow up to 20% of RAM for compressed page pool
echo 20 > /sys/module/zswap/parameters/max_pool_percent

# Enable zswap
echo Y > /sys/module/zswap/parameters/enabled

Persist via /etc/default/grub (or /etc/modprobe.d/zswap.conf):

GRUB_CMDLINE_LINUX_DEFAULT="... zswap.enabled=1 zswap.compressor=zstd zswap.zpool=z3fold zswap.max_pool_percent=20"

Update grub and initramfs:

update-grub

---

Step 4: Calibrating MariaDB & PHP-FPM Configuration

Now align the application configurations to match the cgroup memory boundaries.

MariaDB (/etc/mysql/mariadb.conf.d/50-server.cnf or /etc/my.cnf)

[mysqld]
# Buffer Pool sized within cgroup MemoryLow allocation (12G on a 32G host)
innodb_buffer_pool_size = 12G
innodb_buffer_pool_instances = 8

# Avoid OS page caching double-buffering
innodb_flush_method = O_DIRECT

# Prevent explosive thread memory allocation
max_connections = 250
thread_cache_size = 64
thread_stack = 256K

# Per-connection sort and join limits (prevent runaway queries)
sort_buffer_size = 2M
join_buffer_size = 2M
read_rnd_buffer_size = 1M
tmp_table_size = 64M
max_heap_table_size = 64M

PHP-FPM Pool Configuration (/etc/php/8.3/fpm/pool.d/www.conf)

Tune PHP-FPM workers to guarantee total allocation fits within MemoryHigh (10G):

$$\text{max_children} = \frac{\text{Available Cgroup Memory (10240 MB)} - \text{Base Overhead (1024 MB)}}{\text{Average Worker Memory Footprint (75 MB)}} \approx 120$$

[www]
pm = dynamic
pm.max_children = 120
pm.start_servers = 24
pm.min_spare_servers = 16
pm.max_spare_servers = 32

# Recycle workers to prevent memory fragmentation and slow leaks
pm.max_requests = 1000

# Prevent individual request runaway memory consumption
php_admin_value[memory_limit] = 128M

Restart services:

systemctl restart mariadb php8.3-fpm

---

7. Verification, Benchmarking & PSI Monitoring Script

To ensure your production stack operates without stalls under extreme traffic bursts, deploy this continuous PSI monitoring daemon /usr/local/bin/psi-monitor.sh:

cat <<'EOF' > /usr/local/bin/psi-monitor.sh
#!/usr/bin/env bash
# Real-Time cgroup v2 PSI & Memory Reclaim Monitor
# Nextgen Hosting Systems Engineering

set -euo pipefail

SERVICES=("mariadb.service" "php8.3-fpm.service")

printf "%-22s %-12s %-12s %-10s %-10s %-10s\n" "SERVICE" "MEM CURRENT" "MEM HIGH" "SOME 10s" "FULL 10s" "OOM EVENTS"
printf "%s\n" "--------------------------------------------------------------------------------"

for SVC in "${SERVICES[@]}"; do
    CG_PATH="/sys/fs/cgroup/system.slice/${SVC}"
    if [ ! -d "${CG_PATH}" ]; then
        continue
    fi
    
    CUR_BYTES=$(cat "${CG_PATH}/memory.current" 2>/dev/null || echo 0)
    HIGH_BYTES=$(cat "${CG_PATH}/memory.high" 2>/dev/null || echo "max")
    
    CUR_MB=$((CUR_BYTES / 1024 / 1024))
    if [ "${HIGH_BYTES}" != "max" ]; then
        HIGH_MB=$((HIGH_BYTES / 1024 / 1024))
    else
        HIGH_MB="max"
    fi
    
    SOME_10=$(grep "some" "${CG_PATH}/memory.pressure" | awk '{print $2}' | cut -d'=' -f2)
    FULL_10=$(grep "full" "${CG_PATH}/memory.pressure" | awk '{print $2}' | cut -d'=' -f2)
    OOM_COUNT=$(grep "oom_kill " "${CG_PATH}/memory.events" | awk '{print $2}')
    
    printf "%-22s %-10s MB %-10s MB %-10s %-10s %-10s\n" "${SVC}" "${CUR_MB}" "${HIGH_MB}" "${SOME_10}%" "${FULL_10}%" "${OOM_COUNT}"
done
EOF

chmod +x /usr/local/bin/psi-monitor.sh

Execute the monitor during traffic bursts:

/usr/local/bin/psi-monitor.sh

Sample Post-Tuning Output:

SERVICE                MEM CURRENT  MEM HIGH     SOME 10s   FULL 10s   OOM EVENTS
--------------------------------------------------------------------------------
mariadb.service        12480      MB 24576      MB 0.00%      0.00%      0         
php8.3-fpm.service     6820       MB 10240      MB 0.82%      0.00%      0         

---

8. Summary Configuration Reference Matrix

Metric / Directive	Default Setting	Hardened Production Value	Impact & Architectural Purpose
vm.watermark_scale_factor	10 (0.1%)	200 (2.0%)	Wakes kswapd early; prevents drop into synchronous direct reclaim.
vm.min_free_kbytes	Auto (~64 MB)	262144 (256 MB)	Protects atomic kernel memory allocations and NIC ring buffers.
vm.vfs_cache_pressure	100	50	Preserves dentry and inode cache to avoid filesystem lookup stalls.
MemoryMin (MariaDB)	Not set	12G (Host-dependent)	Hard memory reservation; kernel will never reclaim these pages.
MemoryHigh (PHP-FPM)	Not set	10G (Host-dependent)	Smooth kernel throttling threshold prior to hard OOM limits.
OOMScoreAdjust (MariaDB)	0	-800	Immune to kernel and systemd-oomd premature kill signals.
ManagedOOMMemoryPressure	kill	ignore (MariaDB) / kill (PHP)	Protects transactional database engine while recycling web workers.
Zswap Compression	lzo	zstd + z3fold	Sub-microsecond in-RAM anonymous page compression, avoiding disk I/O.

---

Conclusion & Architecture Roadmap

By moving from unmanaged global memory overcommit to structured systemd cgroup v2 slice isolation, system administrators eliminate the hidden bottleneck of direct reclaim stalls.

MariaDB operates within a protected, non-reclaimable memory zone (MemoryMin/MemoryLow), while PHP-FPM is bounded with MemoryHigh throttles and high-speed Zswap compression. Widening the kernel memory watermark via vm.watermark_scale_factor ensures that background kswapd routines absorb concurrency spikes before worker threads lock up.

For enterprise e-commerce platforms, mission-critical WooCommerce stores, and high-concurrency SaaS applications requiring guaranteed RAM isolation and ultra-low I/O latency, explore Nextgen Hosting Cloud VPS and Enterprise Dedicated Bare-Metal Servers optimized with modern Linux 6.x kernels and hardware NVMe storage.