HIGH CPU USAGE + low memory usage

Question

As you can see in below, Apache is using a high amount of CPU.

 PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 28577 apache 15 0 99676 53m 3488 S 21 0.2 1:13.67 httpd 28568 apache 15 0 99676 53m 3496 S 19 0.2 1:14.92 httpd 28608 apache 15 0 99676 53m 3428 R 19 0.2 0:28.28 httpd 28615 apache 15 0 99676 53m 3436 R 19 0.2 0:25.33 httpd 28616 apache 15 0 99676 53m 3440 S 19 0.2 0:25.83 httpd 28619 apache 15 0 99676 53m 3436 R 19 0.2 0:26.12 httpd 28635 apache 15 0 97.9m 54m 3416 S 19 0.2 0:24.86 httpd 28558 apache 15 0 97.9m 54m 3432 R 17 0.2 1:40.75 httpd 28560 apache 15 0 97.9m 54m 3496 R 17 0.2 1:40.02 httpd 28621 apache 15 0 97.9m 54m 3420 S 17 0.2 0:25.61 httpd 28641 apache 16 0 97.9m 54m 3428 R 17 0.2 0:21.52 httpd 28642 apache 15 0 99756 53m 3424 R 15 0.2 0:21.46 httpd 28643 apache 15 0 99676 53m 3424 S 15 0.2 0:21.59 httpd 28594 apache 15 0 99756 53m 3428 R 13 0.2 0:44.41 httpd 28618 apache 15 0 99676 53m 3420 S 13 0.2 0:26.15 httpd 28654 apache 15 0 99676 53m 3472 S 13 0.2 0:04.27 httpd 28575 apache 15 0 99756 53m 3436 R 11 0.2 1:14.02 httpd 28576 apache 15 0 99676 53m 3496 S 11 0.2 1:16.79 httpd 28634 apache 15 0 99676 53m 3436 S 11 0.2 0:25.36 httpd 28653 apache 15 0 99676 53m 3424 S 11 0.2 0:04.35 httpd 28574 apache 15 0 99676 53m 3440 S 10 0.2 1:13.05 httpd 28592 apache 15 0 99676 53m 3492 R 10 0.2 0:45.78 httpd 28595 apache 15 0 99676 53m 3432 R 10 0.2 0:47.02 httpd 28617 apache 16 0 99676 53m 3436 S 10 0.2 0:25.32 httpd 28620 apache 15 0 99676 53m 3432 S 10 0.2 0:25.35 httpd 28597 apache 15 0 99676 53m 3428 S 8 0.2 0:43.56 httpd 11345 mysql 15 0 2927m 198m 4472 R 4 0.6 1624:43 mysqld 1 root 15 0 2036 648 552 S 0 0.0 0:16.97 init 2 root RT 0 0 0 0 S 0 0.0 0:48.50 migration/0 3 root 34 19 0 0 0 S 0 0.0 0:26.72 ksoftirqd/0 4 root RT 0 0 0 0 S 0 0.0 0:00.00 watchdog/0 5 root RT 0 0 0 0 S 0 0.0 0:04.98 migration/1 6 root 34 19 0 0 0 R 0 0.0 0:27.51 ksoftirqd/1 7 root RT 0 0 0 0 S 0 0.0 0:00.00 watchdog/1 8 root RT 0 0 0 0 S 0 0.0 0:15.42 migration/2 9 root 34 19 0 0 0 S 0 0.0 0:26.50 ksoftirqd/2 10 root RT 0 0 0 0 S 0 0.0 0:00.00 watchdog/2
 

How should I go about profiling what is causing this high CPU usage and what can I do to decrease it?

Answer 1

Without more information the two most likely reasons for high CPU load from Apache are simply:

1. High Traffic - A lot of incoming requests equates to a lot of Apache processes doing work.
2. Slow PHP Scripts - Slow, poorly optimized, or even buggy PHP scripts can hog a lot of CPU quickly.

I would setup mod_status to enable the extended '/server-status' page which will tell you a lot more detail on what Apache is currently serving. From there you may be able to see if it is one client hammering the site, if it is one particular page that is timing out, or if too many requests are just beginning to bog down the site. There are many potential root causes and each has a different solution so I would try to find or narrow down the cause first.