Server memory
Reserved for other services (MB)
Headroom for traffic spikes and memory growth. 10–20% is typical; max 90% (higher values are reset to 90).
Measure it on your server (average RSS of all php-fpm processes):
ps --no-headers -o "rss,cmd" -C php-fpm | awk '{ sum+=$1 } END { printf ("%d%s\n", sum/NR/1024,"M") }'
Debian/Ubuntu: use -C php-fpm8.3 (your PHP version). Run it under real traffic.
Recommended pool size
38
pm.max_children
pm.start_servers9
pm.min_spare_servers9
pm.max_spare_servers28
Total RAM
Reserved (OS + DB + Redis + other)
Usable for PHP-FPM (after buffer)
Peak PHP memory at max_children
Formula
pm.max_children = floor((RAM − reserved) × (1 − buffer) ÷ worker_size)
Process manager mode & www.conf
Pool config (www.conf)