How to flush cache in Linux

Hello guys,

Me again with a very interesting problem. Today I was working on some tasks as usual and I discovered that something is wrong with one of our systems and the vehicles are not updated actively. Something went wrong with the MQTT connection and that’s how I arrived to write How I found a simple UI Viewer for InfluxDB article.

While I debugged the problem I observed that the server’s RAM memory is full, but guess what, the memory used was 489640 and free memory 359256 from a total of 4GB. That’s why I came with the great idea As you can see below, the buff/cache memory is above 3GB (3189056).

$ top


top - 13:42:55 up 168 days,  5:26,  1 user,  load average: 0.00, 0.00, 0.00
Tasks: 109 total,   1 running,  67 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.3 us,  0.0 sy,  0.0 ni, 99.5 id,  0.0 wa,  0.0 hi,  0.0 si,  0.2 st
KiB Mem :  4037952 total,   359256 free,   489640 used,  3189056 buff/cache
KiB Swap:        0 total,        0 free,        0 used.  3268392 avail Mem 

Check the cache memory

$ free -m

              total        used        free      shared  buff/cache   available
Mem:           3943         503         365           1        3074        3167
Swap:             0           0           0

Clear the cache memory

$ sync; echo 3 | sudo tee /proc/sys/vm/drop_caches

Now you can check again the cache, and surprise

$ free -m

              total        used        free      shared  buff/cache   available
Mem:           3943         505        3123           1         314        3206

And this is how it looks now my result of top command

$ top

top - 13:46:19 up 168 days,  5:29,  1 user,  load average: 0.00, 0.00, 0.00
Tasks: 110 total,   1 running,  67 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.0 us,  0.2 sy,  0.0 ni, 99.8 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem :  4037952 total,  3196540 free,   517812 used,   323600 buff/cache
KiB Swap:        0 total,        0 free,        0 used.  3282840 avail Mem 

Automate this with a cronjob

I know that you’re lazy, so do I. Now let’s go back in business and create a Cronjob that automates this process for you.

$ crontab -e

A new text editor will open now, probably it’s VIM. All you have to do is to add at the end of file the following:

# m h  dom mon dow   command

0 2 * *  * sync; echo 3 | sudo tee /proc/sys/vm/drop_caches

This cron will execute every day at 2am.

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