Linux has a memory management system that will use free RAM as cache. This section of memory should be freely available to be taken over when something else needs it, but it seems like Linux is not releasing it because it thinks it is being used and this causes problems. I monitor our servers and recently I’ve been having issues with memory showing critical.
Whenever this happens, we would usually wait until it get’s really slow and then restart. I got tired of restarting the server every time so I set out researching and found an easy way.
Log into your troubled machine and become root
Run one of the following, I use the echo 3 option:
- To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
- To free dentries and inodes:
echo 2 > /proc/sys/vm/drop_caches
- To free pagecache, dentries and inodes:
echo 3 > /proc/sys/vm/drop_caches
Log into your troubled machine and create a script called clearcache.sh with the following content:
#!/bin/sh sync; echo 3 > /proc/sys/vm/drop_caches
Setup a crontab entry:
sudo -i crontab -e
Scroll to the bottom of the file and enter the following line
0 * * * * /path/to/clearcache.sh
Create a file in ‘/root’ called ‘clearcache.sh’ with the following content
Once you have saved this file, the job is complete!
Every hour the cron job will run this command and clear any memory cache that was built up.