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Hi pbuchbinder and thanks for this useful module.
I've used it for quite some time on multiple machine and never had a problem so far.
On a busy reverse proxy with mod_qos when there is a connection spike, apache is shutting down with "Child XXXXX returned a Fatal error… Apache is exiting!". Other interesting errors: " (22)Invalid argument: ap_queue_info_set_idle failed. Attempting to shutdown process gracefully." and lots of " (22)Invalid argument: ap_queue_pop failed" (more logs below).
First I didn't suspected mod_qos but some os limitation (due to " (11)Resource temporarily unavailable: apr_thread_create: unable to create worker thread" so I've dubled the stack size (ulimit -s 20480), halfed ThreadsPerChild and fine tuned ThreadLimit to no avail.
On the other hand, there are several mod_qos errors (" mod_qos(008): could not create supervisor thread (create thread), disable request rate enforcement" ) and it never happend after disabling mod_qos.
The machine is a linux 64bits centos 6.2 server running apache 2.2.22 with mod_qos 10.2, mod_security 2.6.3 and mod_disk_cache / mod_proxy / mod_proxy_http / mod_proxy_balancer .
More info on pastebin (expireing in 3 weeks, password is mod_qos):
full apache error log: http://pastebin.ca/J82y5Z-j
mod_qos conf: http://pastebin.ca/HMmuFoWV
other relevant apache conf: http://pastebin.ca/N8puMLsd
machine info: http://pastebin.ca/SJMBGQaM
other info available on request
The "apr_thread_create: unable to create worker thread" message sounds like a memory issue (the newly forked child process - created to due to the MaxRequestsPerChild settings - is not able to allocate the memory for all 32+ threads). Your setting allows up to 256 child processes. I don't know how much memory each of theses processes need, but it might be quite much…