Mysql – Tuning Apache/MySQL/PHP for WordPress

apache-2.2MySQLWordpress

I am trying to figure out why my apache processes are eating so much memory

My slice specs( 1.5GB RAM, CentOS 5, Apache2, PHP 5.2, MySQL)

As you can see my top processes are consuming nearly half of my entire memory
and when more processes are spawned the server nearly grinds to a halt,
frequently going over into swap and crashing.

PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND   
6817 apache    40   0  296m 103m 3920 S  0.0  6.7   0:03.52 httpd                         
6789 apache    40   0  295m 101m 3932 S  0.0  6.6   0:07.04 httpd                         
6765 apache    40   0  284m  91m 3948 S 55.1  5.9   0:12.45 httpd                         
6798 apache    40   0  284m  90m 3944 S  0.0  5.9   0:05.49 httpd                         
6542 apache    40   0  283m  90m 3956 S  0.0  5.8   0:43.25 httpd                         
6827 apache    40   0  283m  88m 3796 S  0.0  5.7   0:01.83 httpd

Does anyone have any clue what could be causing apache (and php) to be consuming so much memory?

              total       used       free     shared    buffers     cached
Mem:          1545        827        718          0          3        111
-/+ buffers/cache:        713        832
Swap:         3071        103       2968

Best Answer

You're almost certainly running mod_php, which means you're almost certainly running apache in mpm_prefork.

If performance is generally okay until load causes you to start swapping, a quick fix is to start throttling down apache's MaxClients. If apache is allowed to fork worker processes whenever it wants, it's going to start swapping under load. Requests will queue until a worker is available, so things can get slow, but not as slow as the death-by-swap.

If you really need to tune things tighter, consider getting away from prefork apache mpm. That means running PHP as FastCGI. If you're going to go with PHP under FastCGI, you should consider upgrading to PHP 5.3.3, which has much nicer FastCGI process manager (--enable-fpm configure option).

php-fpm/worker is much more memory efficient than old-fashioned mod_php. You can then tune the number of apache processes/threads independently of the number of PHP processes. And your memory-heavy PHP processes are only used for serving up php-driven content, and not wasted on serving static files.

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