Optimal values for ServerLimit, MaxClients, MaxRequestsPerChild directives

apache-2.2optimizationperformance

I'm running a traffic intense site plenty of dynamic content, mostly user-generated.

The server is a dedicated one and has a total of 4 Intel(R) Xeon(R) CPU X3210 @ 2.13GHz proccesors. I need to know to optimal values for ServerLimit and MaxClients apache's directives, considering that the server has 4GB of RAM and the MySQL database runs on a separate server. The panel is DirectAdmin with CentOS.

Below are my current directives, but during peak hours with over 5k users, an important lag is noticed – and it's not entirey MySQL's fault, because pages seem to be generated fast (I implemented a page generation time counter), but there is a long connection delay until the page starts responding and is sent to the browser.

<IfModule prefork.c>
    StartServers     800
    MinSpareServers   20
    MaxSpareServers   60
    ServerLimit      900
    MaxClients       900
    MaxRequestsPerChild  2000
</IfModule>
Timeout 90
KeepAlive On
KeepAliveTimeout 5

I should mention that monitoring the server using the top command, CPU usage never goes beyond 20% ~ 30% on peak hour. The MySQL server also has a 30~50% usage at that time, and I'm constantly working on fixing slow queries, but that's a different issue. I know it's not a DB bottleneck because static pages also take long to load on peak hours.

Any tips to optimize these values will be greatly appreciated, thanks.

Best Answer

Your MaxClients is WAY WAY WAY too high. What is the current size of your apache process? Multiply that x 900. Is that greater than 4GB? If so, the machine is likely going into swap. I usually start with MaxClients = 2x vCPUs in the box (grep -c processor /proc/cpuinfo). Which in this case would be about 8. Then make sure that MaxClients x apache process size isn't over 4GB.

You can up your MaxClients from there, depending on the type of connection that your clients have. (Dial-up users need to be spoonfed, etc.) But make sure you never get yourself into a swapping situation.

Then set your Min, Max, and Start servers to MaxClients. There's no real need to have them differ in a dedicated server environment.

Then do some testing with ab (as goose notes.)

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