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PHP-FPM: [pool www] server reached pm.max_children setting, consider raising it

All PHP workers of the pool are busy. New requests queue until one is free, so pages slow down and nginx may return 502 or 504. The cause is either too few workers for the load, or workers that are stuck on something slow.

daniele@mikweb: ~/errors — less php-fpm-server-reached-pm-max-children.md

What the message looks like

[29-Oct-2026 10:14:52] WARNING: [pool www] server reached pm.max_children setting (5), consider raising it

Common causes

  • Slow scripts keep workers busy for long: a slow database query, a slow external API, a lock.
  • A burst of traffic or bots hits uncached PHP pages.
  • pm.max_children is simply too low for the normal load of this server.
  • The value was raised too far in the past and PHP now exhausts RAM, so workers get killed instead (check the OOM page).
  • Long-running requests such as exports or imports block workers.

Check

  1. Active pool settings (without comments)
    grep -hE '^(pm|pm\.[a-z_]+) *=' /etc/php/*/fpm/pool.d/*.conf
  2. How often did the pool hit the limit?
    journalctl -u 'php*-fpm' --no-pager | grep -c 'max_children'
  3. Average memory per PHP-FPM worker
    ps -eo rss,cmd | awk '/[p]hp-fpm: pool/ {s+=$1; n++} END {if (n) print n " workers, " s/n/1024 " MB average"}'
  4. Free memory on the server
    free -m

Fix

  • Size the pool from memory: pm.max_children = RAM you can give to PHP divided by the average worker size from the check above.
  • Set it in the pool file under /etc/php/<version>/fpm/pool.d/www.conf, together with matching pm.start_servers, pm.min_spare_servers and pm.max_spare_servers for pm = dynamic.
  • Find the slow requests: set request_slowlog_timeout = 5s and slowlog = /var/log/php-fpm-slow.log in the pool, then read the stack traces.
  • Reduce the load: cache pages, block abusive bots, move long jobs out of web requests.
  • Restart the PHP-FPM service of your version, then watch the log for the warning.

FAQ

Can I just set a very high pm.max_children?

No. Each worker uses memory. If the total exceeds RAM, the kernel kills processes or the server swaps, which is worse than queued requests.

Which pm mode should I use?

dynamic is a good default. ondemand saves RAM on idle servers but is slower on first requests, static is for busy servers with predictable memory.

daniele@mikweb: ~/errors — ls

More errors

Call to undefined function mysqli_connect()PHP Fatal error: Uncaught Error: Call to undefined function mysqli_connect()Cannot modify header information - headers already sentPHP Warning: Cannot modify header information - headers already sent by (output started at ...)The uploaded file exceeds the upload_max_filesize directive in php.iniPHP: The uploaded file exceeds the upload_max_filesize directive in php.iniAllowed memory size exhaustedPHP Fatal error: Allowed memory size exhaustedMaximum execution time exceededPHP Fatal error: Maximum execution time of 30 seconds exceededcannot load certificatenginx: [emerg] cannot load certificate (PEM no start line, key values mismatch)

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