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Mr.Boon
Asked: 2012-01-19 09:27:33 +0800 CST

Memory usages in Linux drops frequently

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I run a CentOS 5.6 (64bit) machine that has Nginx (latest version) running, with php-fpm (latest version). Things run very well, but since about 2 weeks I noticed in my Munin graphs that about every 2 hours the 'cache' usages drops. Before it used be a steady fully graph, that didn't seem to reset every so often.

PHP-FPM settings:

pm.max_children = 300
daemonize = yes
pm = static
listen = /tmp/fpm.sock
pm.max_requests = 1000

I have checked the php-fpm.log, and about once per 5 seconds a child process is killed, and restarted. But this is all the time, so this does not explain the sudden drops.

munin graph

I only run Nginx, PHP (via fpm), Munin and vsftpd on this machine. No crons run at exactly the time of the drops.

My question: What could be causing these drops in cache usage?

centos memory php-fpm

3 Answers

  1. I think some large file that's part of your webserver is responsible. I'm guessing a log that is rotated or compressed as a function of general housekeeping. Below are some experiments showing that Linux dumps that part of the cache when a file is removed, including by means of a utility like gzip.

    $ free
             total       used       free     shared    buffers     cached
Mem:       4054352    3867292     187060          0      75664     390788
-/+ buffers/cache:    3400840     653512
Swap:      4194300     338344    3855956

    # echo 1 > /proc/sys/vm/drop_caches
$ free
             total       used       free     shared    buffers     cached
Mem:       4054352    3420612     633740          0        292      20516
-/+ buffers/cache:    3399804     654548
Swap:      4194300     338344    3855956

    But that's probably not what's happening. What if a file were deleted? Would it still be referenced?

    $ dd if=/dev/zero of=cachetest bs=1M count=200
200+0 records in
200+0 records out
209715200 bytes (210 MB) copied, 0.288883 s, 726 MB/s

$ free
             total       used       free     shared    buffers     cached
Mem:       4054352    3647688     406664          0       3200     240328
-/+ buffers/cache:    3404160     650192
Swap:      4194300     338344    3855956

$ rm cachetest
$ free

             total       used       free     shared    buffers     cached
Mem:       4054352    3438508     615844          0       3208      35528
-/+ buffers/cache:    3399772     654580
Swap:      4194300     338344    3855956

    And, if we reload it then compress the file:

    $gzip cachetest
$free
             total       used       free     shared    buffers     cached
Mem:       4054352    3439412     614940          0       3384      36404
-/+ buffers/cache:    3399624     654728
Swap:      4194300     338344    3855956
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  2. Best Answer

    I'm not sure about CentOS 5.6, but for my Linux 2.6.32-5-amd64 adding this line in /etc/sysctl.conf

    vm.vfs_cache_pressure = 1

    resolves this problem.

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  3. Solved it by putting vm.zone_reclaim_mode = 0

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