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How to reduce PHP-FPM (php5-fpm) RAM usage by about 50%

I became aware of what an alternative configuration would do after reading an article titled A better way to run PHP-FPM. It was written about a year ago, so it’s kinda disappointing that I came across it while searching for a related topic just last night. If you run your own server and use PHP with PHP-FPM, you need to read that article.

After I read it, I changed the pm options in the pool configuration file to these:

Updated pool PHP-FPM values

Shell

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; Choose how the process manager will control the number of child processes. pm = ondemand pm.max_children = 75 pm.process_idle_timeout = 10s pm.max_requests = 500

The major change was setting pm = ondemand instead of pm = dynamic. And the impact on resource usage was drastic. Here, for example, is the output of

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free –mt after reloading php5-fpm:

Output of `free -mt`

Shell

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total       used       free     shared    buffers     cached Mem:           490        196        293         28          9         70 –/+ buffers/cache:        116        373 Swap:         2047        452       1595 Total:        2538        649       1888

Compared to the output before, that’s more than a 50% drop in RAM usage. And the reason became obvious when I viewed top again:

Output of `top`

Shell

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2778 mysql     20   1359152  56708   3384 S  0.0 11.3   2:11.06 mysqld                               26896 root      20     373828  19000  13532 S  0.0  3.8   :02.42 php5–fpm                             25818 root      20      64208   4148   1492 S  0.0  0.8   :01.88 php5–fpm 25818 root      20      64208   4148   1492 S  0.0  0.8   :01.88 php5–fpm                             17385 root      20      64208   4068   1416 S  0.0  0.8   :02.23 php5–fpm                              1465 ossec     20      15592   2960    480 S  0.0  0.6   :08.60 ossec–analysisd                       1500 root      20       6312   2072    328 S  0.0  0.4   :45.55 ossec–syscheckd                          1 root      20      33444   1940    812 S  0.0  0.4   :03.29 init

Did you notice that there are no child processes? What happened to them? That’s what setting pm = ondemand does. A child process is spawned only when needed. After it’s done its job, it remains idle for 10 seconds (pm.process_idle_timeout = 10s) and then dies.

So what I have is a simple modification to the default PHP-FPM settings that saved me more than 50% of RAM. Sure, the server hasn’t come under heavy traffic, but I think it can withstand a reasonably heavy traffic, considering that it only has 512 MB of RAM. And with Nginx microcaching configured, I think it will do very well. There are other aspects of PHP-FPM and Percona MySQL that I’ve not optimized yet, so stay tuned. This was just to pass on a little tip that I found useful.