FPM Status: active processes greater than pm.max_children

Description

Given the following FPM Pool configuration the Pool is configured using 1500 static workers and a listen.backlog of 100:

$ cat /etc/php/8.3/fpm/pool.d/backoffice.conf 
[backoffice]
listen = /run/php8.3-fpm_backoffice.sock
listen.backlog = 100

pm = static
pm.max_children = 1500
pm.status_path = /php8.3-fpm-status-backoffice.php

; The number of requests each child process should execute before respawning. This can be useful to work around memory leaks
pm.max_requests = 2000

The listen.backlog works like expected on the socket.

$ ss -lxn src ==  /run/php8.3-fpm_backoffice.sock
Netid                      State                       Recv-Q                      Send-Q                                                              Local Address:Port                                              Peer Address:Port                      Process
u_str                      LISTEN                      0                           100                                               /run/php8.3-fpm_backoffice.sock 47129                                                        * 0

Without high traffic total processes reports 1500 as expected.

$ curl -s 127.0.0.1:80/php8.3-fpm-status-backoffice.php
pool:                 backoffice
process manager:      static
start time:           13/May/2024:04:02:02 +0200
start since:          22533
accepted conn:        3057525
listen queue:         0
max listen queue:     0
listen queue len:     0
idle processes:       1460
active processes:     40
total processes:      1500
max active processes: 549
max children reached: 0
slow requests:        0

When generating high concurrent requests using a tool like siege or k6 I can observe the following output from the status page:

$ curl 127.0.0.1:80/php8.3-fpm-status-backoffice.php
pool:                 backoffice
process manager:      static
start time:           08/May/2024:04:02:01 +0200
start since:          19565
accepted conn:        10120636
listen queue:         0
max listen queue:     0
listen queue len:     0
idle processes:       1
active processes:     3347
total processes:      3348
max active processes: 3503
max children reached: 0
slow requests:        0

active processes/total processes/max active processes are way over the defined pm.max_children = 1500.

How can active processes be greater than pm.max_children?

Also max children reachedreports 0.

Relates to: https://github.com/php/php-src/issues/9943#issuecomment-1607206535

PHP Version

PHP 8.3.6

Operating System

Debian 11