What is pm.max_children?
This is the maximum number of simultaneous PHP processes that the pool can execute.
Each process = 1 active PHP request. Each process consumes RAM.
👉 If the limit is exceeded → error 502 / timeout
👉 If it gets excessive → swap / OOM / slow server
1️⃣ Main rule (actual formula)
pm.max_children = RAM available for PHP
÷
average resource consumption of a 1 PHP process
Simple, but you need to measure it correctly.
2️⃣ Find out how much RAM is available to PHP.
Step 1 – Total RAM
free -m
Example:
Total: 12288 MB
Step 2 – Reserve RAM for the system
Always book:
- OS + services: 2–3 GB
- MySQL/MariaDB: 30–40% of RAM
- Cache (Redis, OPcache): 500 MB – 1 GB
Real-life example (12 GB RAM):
Total: 12288 MB System: 2048 MB MariaDB: 4096 MB Cache/Others: 512 MB ------------------------- Available PHP: ~5632 MB
3️⃣ Measure the actual power consumption of a PHP-FPM process.
Correct method (production)
ps --no-headers -o "rss,cmd" -C php-fpm | awk '{sum+=$1} END {print sum/NR/1024 " MB"}'
Example output:
42 MB
📌 In WordPress:
- PHP simple: 30–40 MB
- WooCommerce: 50–80 MB
- Sites heavy: 100+ MB
4️⃣ Calculate the pm.max_children
Using the example:
5632 MB ÷ 42 MB ≈ 134 processes
Safe value:
pm.max_children = 120
📌 Always leave a 10–15% margin.
5️⃣ Choose the correct PM (this changes everything)
🔹 ondemand (RECOMMENDED)
pm = ondemand pm.max_children = 120 pm.process_idle_timeout = 10s
✔ Less RAM
✔ Ideal for WordPress
✔ Prevents idle processes
🔹 dynamic
pm = dynamic pm.max_children = 120 pm.start_servers = 10 pm.min_spare_servers = 5 pm.max_spare_servers = 20
⚠️ It uses more RAM (always-on processes).
6️⃣ Environments with CloudLinux (LVE)
👉 The calculation should be per account, not global.
Example per user:
- LVE RAM: 1 GB
- PHP Process: 40 MB
1024 ÷ 40 ≈ 25
pm.max_children = 20
📌 This prevents a website from crashing the server.
7️⃣ Signs of a misconfigured pm.max_children error.
Very low
- Errors 502
server reached pm.max_children- Slowness under load
Very high
- Swap active
- OOM Killer
- Server slow even with no traffic
8️⃣ Real-time monitoring
watch -n1 "ps -ylC php-fpm --sort:rss | head"
View errors:
journalctl -u php-fpm
9️⃣ Exemplo final (produção WordPress)
Server:
- 12 GB RAM
- MariaDB 11.x
- CloudLinux
- Nginx + Apache
pm = ondemand pm.max_children = 120 pm.process_idle_timeout = 10s pm.max_requests = 500
✅ Quick checklist
✔ RAM measured correctly
✔ Actual PHP usage calculated
✔ Safety margin applied
✔ pm=ondemand used
✔ Active monitoring