Soheil's questions

I'm using apache mpm event on centos server with the following configs:

<IfModule event.c>
StartServers 8
ServerLimit 64
ThreadsPerChild 256
MaxRequestWorkers 16384
MaxConnectionsPerChild 10000
MinSpareThreads 125
MaxSpareThreads 250
ThreadLimit 256
KeepAlive On
KeepAliveTimeout 2
MaxKeepAliveRequests 500
</IfModule>

I then increased the value of StartServers from 8 to 32 and rebuilt and restarted apache, but I can't see any difference in free memory of system. I expected to see more ram is being used by apache when I increase the value of startServers. why no change is happening in ram usage? does it mean I'm doing something wrong?

I'm using apache MPM event on my centos server with these configs:

<IfModule event.c>
StartServers 8
ServerLimit 32
ThreadsPerChild 256
MaxRequestWorkers 8192
MaxConnectionsPerChild 10000
MinSpareThreads 125
MaxSpareThreads 250
ThreadLimit 256
KeepAlive On
KeepAliveTimeout 2
MaxKeepAliveRequests 500
</IfModule>

now what I need is to check what number of servers and threads are currently created by apache. how can I check these values?

I'm using apache mpm event and php-fpm on my CentOS 8 server. I need to be able to make http requests from my server to a php api on my server, so the url of the api would be something like: http://127.0.0.1/~<account_name>/api.php. there is no problem with the url and the php file gets found on the server, but it generates a 501 Not Implemented error.

what is causing this error when I try to make http requests to localhost and how to solve the issue?

I'm totally confused about the memory metrics provided by free command in linux. currently I receive the following metrics:

              total        used        free      shared  buff/cache   available
Mem:          1.9Gi       624Mi       759Mi       8.0Mi       603Mi       1.2Gi

1. it is showing 759Mi free memory which is relatively large. it is defined to be the unused memory and available memory is defined to be the memory available for starting new app without swapping. how available is larger than free? and is unused part of available memory?
2. buff/cache is 603Mi. as I understand it is also counted in available memory. but how? suppose I want to start an app which needs 1.2Gi memory (entire available memory), then what happens to the data currently in buff/cache? they get cleared from RAM? and in this case how the system continues to do buffering and caching without having any memory left?!