Tuning Apache2 prefork MaxClients ServerLimit

Question

I have a machine with 128 GB Ram that is using Apache2 as Webserver (in this machine there is no Database Server, the Database Machine is a 64 GB Ram machine that can handle 2000 max connections). I see with a monitoring tool that there are at the moment about 44 busy workers and 12 idle workers, what are best theoretical values for my prefork module?

i got blank pages sometimes loading websites on high load hours and got this error on my apache error log:

[notice] child pid 13595 exit signal Segmentation fault (11)

how can solve this issue too?

My Apache2 Prefork Module configuration:

StartServers          3
MinSpareServers       3
MaxSpareServers       5
ServerLimit           3200
MaxClients            3100
MaxRequestsPerChild   0

Free -h on the www Machine :

total: 128 G free: 97GB (with apache2 running) shared 0b buffers 1.9G cache 23G

Ram used By Apache2 and other Programs:

Private  +   Shared  =  RAM used    Program

 96.0 KiB +  61.0 KiB = 157.0 KiB   sh
176.0 KiB +  26.0 KiB = 202.0 KiB   atd
176.0 KiB +  35.5 KiB = 211.5 KiB   acpid
208.0 KiB +  19.5 KiB = 227.5 KiB   mdadm
204.0 KiB +  30.0 KiB = 234.0 KiB   init
248.0 KiB +  62.0 KiB = 310.0 KiB   sendmail
376.0 KiB +  36.0 KiB = 412.0 KiB   dbus-daemon
388.0 KiB + 285.5 KiB = 673.5 KiB   cron (2)
820.0 KiB +  42.0 KiB = 862.0 KiB   gam_server
920.0 KiB + 108.0 KiB =   1.0 MiB   ntpd
968.0 KiB + 243.0 KiB =   1.2 MiB   getty (6)
  1.3 MiB + 351.5 KiB =   1.6 MiB   udevd (3)
  1.5 MiB + 343.0 KiB =   1.8 MiB   sendmail-msp
  2.0 MiB + 910.0 KiB =   2.9 MiB   plugin-localresources2
  3.4 MiB +  50.0 KiB =   3.4 MiB   rsyslogd
  3.6 MiB +  68.5 KiB =   3.7 MiB   bash
  1.9 MiB +   2.1 MiB =   4.0 MiB   sendmail-mta (4)
  3.8 MiB + 556.0 KiB =   4.3 MiB   sshd (2)
  3.7 MiB +   1.2 MiB =   4.8 MiB   plugin-apache2
  5.1 MiB +   1.2 MiB =   6.3 MiB   agent-service
  7.0 MiB + 654.0 KiB =   7.6 MiB   fail2ban-server
  9.6 MiB +   2.6 MiB =  12.2 MiB   proftpd (8)
 59.2 MiB +  70.0 KiB =  59.3 MiB   miniserv.pl
 96.8 MiB +   3.6 MiB = 100.4 MiB   php5-cgi (2)
196.4 MiB +  35.9 MiB = 232.3 MiB   apache2 (40)
---------------------------------
                     tot 450.0 MiB

Answer 1

Apache prefork settings, per apache performance tuning guidelines

quote:

The single biggest hardware issue affecting webserver performance is RAM.
A webserver should never ever have to swap, as swapping increases the latency
of each request beyond a point that users consider "fast enough". 
This causes users to hit stop and reload, further increasing the load.
You can, and should, control the MaxClients setting so that your server does
not spawn so many children it starts swapping. This procedure for doing this
is simple: determine the size of your average Apache process, by looking at
your process list via a tool such as top, and divide this into your total 
available memory, leaving some room for other processes.

you should set it up like this based on your input to:

• Total Memory : 128 GB
• -10% memory for everything except apache: 115 GB
• Now we need to figure out how much single apache process is using.

To calculate this you can use following script :

pgrep apache2 | xargs -n1 -I{} cat /proc/{}/smaps | \
  awk '{if ($0 ~ /stack/) {pids+=1} else if ($0 ~/^Shared_/) 
    {shared+=$2} else if ($0 ~ /^Pss:/) {priv+=$2}} END {
      printf "%.2f MB\n",(priv+shared/(pids*pids))/1024}'

This is best estimate of how much single apache process is using memory while trying to proportionally dividing shared usage per number of active apache processes and adding it on top of Pss (proportional set size)

Finally you divide 115 GB with this figure and you get MaxClients/ServerLimit. From here you can relatively calculate other figures like

• StartServers 30% of MaxClients
• MinSpareServers 5% of MaxClients
• MaxSpareServers 10% of MaxClients
• ServerLimit == MaxClients
• MaxConnectionsPerChild 10000 (as conservative alternative to address possible problem with memory leaky apps)