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I rent a Gentoo server with the usual LAMP stack (prefork Apache MPM) + suPHP.

From time to time, my server runs out of memory and slows down to a crawl (responds to pings, but it's practically impossible to log in, and keystrokes sent through SSH can take minutes to be echoed back, much less processed). Lots of oom_killer stuff in the system logs, too.

This is what I see in top during one of these moments:

top - 16:45:05 up 22 days,  8:08,  3 users,  load average: 104.26, 103.87, 93.3
Tasks: 393 total,   1 running, 388 sleeping,   0 stopped,   4 zombie
Cpu(s):  4.6%us,  9.3%sy,  0.8%ni,  0.0%id, 84.8%wa,  0.0%hi,  0.5%si,  0.0%st
Mem:   2042128k total,  1634392k used,   407736k free,     1792k buffers
Swap:        0k total,        0k used,        0k free,    27724k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 3125 apache    20   0  288m 105m 1368 S    0  5.3   0:01.00 apache2
 2886 apache    20   0  285m 102m 1368 S    0  5.1   0:02.44 apache2
 3048 apache    20   0  279m  96m 1192 D    0  4.8   0:01.58 apache2
 3037 apache    20   0  278m  95m 1076 S    0  4.8   0:02.23 apache2
 3014 apache    20   0  278m  94m 1204 D    0  4.8   0:01.81 apache2
 2859 apache    20   0  274m  91m 1368 S    0  4.6   0:00.63 apache2
 3016 apache    20   0  269m  86m 1368 S    0  4.3   0:01.49 apache2
 2887 apache    20   0  269m  86m 1192 D    0  4.3   0:01.06 apache2
 2753 apache    20   0  269m  86m 1368 S    0  4.3   0:01.09 apache2
 3036 apache    20   0  266m  83m 1372 S    1  4.2   0:01.10 apache2
 3006 apache    20   0  266m  83m 1368 S    0  4.2   0:01.98 apache2
 3007 apache    20   0  265m  82m 1372 S    0  4.1   0:02.00 apache2
 3064 apache    20   0  264m  81m 1368 S    0  4.1   0:00.57 apache2
 3045 apache    20   0  263m  80m 1368 S    0  4.1   0:00.60 apache2
 2888 apache    20   0  263m  79m  416 S    0  4.0   0:01.09 apache2
 2862 apache    20   0  260m  77m 1368 S    1  3.9   0:01.95 apache2
 2891 apache    20   0  259m  76m 1332 D    0  3.9   0:01.98 apache2
 3046 apache    20   0  258m  75m 1080 S    0  3.8   0:01.20 apache2
 2873 apache    20   0  255m  72m 1380 S    0  3.6   0:01.51 apache2
 2987 apache    20   0  252m  69m 1368 S    0  3.5   0:01.04 apache2
 2666 apache    20   0  250m  67m 1368 S    0  3.4   0:00.72 apache2
 2903 apache    20   0  248m  66m 1368 S    0  3.3   0:01.02 apache2
 3013 apache    20   0  247m  63m  416 S    0  3.2   0:01.02 apache2

Note that PHP is running in CGI mode, so this is just Apache without any PHP modules.

Frankly I don't understand why else would it be slow other than running oun of RAM, yet it claims it has 400 MB of free RAM. "84.8%wa" also indicates that the system is waiting for I/O operations (paging?).

Things I tried:

  • Disabling swap, with the hope that things that start eating memory rampantly will just crash instead of bringing the server down to a grind - this didn't work, it probably just began paging out memory-mapped files (executables and SOs)
  • Setting oom_adj of the root Apache process to 15
  • Tweaking the MPM settings:

    StartServers         5
    MinSpareServers      5
    MaxSpareServers      10
    MaxClients           50
    MaxRequestsPerChild  10
    MaxMemFree           1024
    

For now I've reduced MaxClients to 25, but now page requests take several seconds to process, and some kid with FlashGet can theoretically clog all Apache processes and effectively make all websites inaccessible :/

Questions:

  1. Can anyone suggest some Apache configuration tweaks which could radically improve my situation?

  2. Is it possible to tell Linux to not swap/page out sshd, bash, and everything else required for me to ssh in and kill runaway processes?

  3. If the answer to the above question is 'no', someone please explain to me how is it that in this day and age modern operating systems are such horrendously flawed. Sounds like epic fail in OS design to me :(

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1 Answer 1

up vote 2 down vote accepted

Your apache processes each use about 80 MB of memory (RES column). This is huge - here I have a web server with apache 2.2 used for running CGIs and each process uses about 5 MB. Now, if you have 50 apache processes, each using 80MB, then you're swapping like hell. Once we find why you are consumming so much memory, we'll likely resolve the issue.

Can you post your apache configuration file ?

share|improve this answer
    
Umm... the configuration is spread out through a lot of files. I think it's a pretty standard Gentoo configuration, with some tweaks. One non-standard module I'm running is mod_security. Do you think it might be causing it? I'll try disabling it and see if it makes a difference... –  CyberShadow Aug 31 '10 at 15:52
    
Heh, I just discovered lots of crap that was enabled but I didn't need [anymore]. Deleted this from /etc/conf.d/apache: "-D SECURITY -D DAV -D SVN -D PYTHON -D LANGUAGE". Now apache2 starts out with 7 "RES" MB instead of 40. Now I'll just need to figure out how to enable this stuff when I do need it for a few vhosts, without affecting system performance globally. –  CyberShadow Aug 31 '10 at 16:01
    
How's the system responsiveness with those changes ? –  Urgoll Aug 31 '10 at 16:11
    
I'd have to wait until someone DoSes me again :P –  CyberShadow Aug 31 '10 at 16:13
1  
worker is in theory better than prefork wrt memory usage, but I'd be careful with 3rd party modules (such as mod_security) to make sure they are fully tested with the worker MPM as multithreading comes with its own bucket of gotchas. –  Urgoll Aug 31 '10 at 17:09

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