How to Clear RAM Memory Cache, Buffer and Swap Space on Linux

Like any other operating system, GNU/Linux has implemented a memory management efficiently and even more than that. But if any process is eating away your memory and you want to clear it, Linux provides a way to flush or clear ram cache.

Clear RAM Cache and Swap in Linux

How to Clear Cache in Linux?

Every Linux System has three options to clear cache without interrupting any processes or services.

1. Clear PageCache only.

# sync; echo 1 > /proc/sys/vm/drop_caches

2. Clear dentries and inodes.

# sync; echo 2 > /proc/sys/vm/drop_caches

3. Clear PageCache, dentries and inodes.

# sync; echo 3 > /proc/sys/vm/drop_caches 

Explanation of above command.

sync will flush the file system buffer. Command Separated by “;” run sequentially. The shell wait for each command to terminate before executing the next command in the sequence. As mentioned in kernel documentation, writing to drop_cache will clean cache without killing any application/service, command echo is doing the job of writing to file.

If you have to clear the disk cache, the first command is safest in enterprise and production as “...echo 1 > ….” will clear the PageCache only. It is not recommended to use third option above “...echo 3 >” in production until you know what you are doing, as it will clear PageCache, dentries and inodes.

Is it a good idea to free Buffer and Cache in Linux that might be used by Linux Kernel?

When you are applying various settings and want to check, if it is actually implemented specially on I/O-extensive benchmark, then you may need to clear buffer cache. You can drop cache as explained above without rebooting the System i.e., no downtime required.

Linux is designed in such a way that it looks into disk cache before looking onto the disk. If it finds the resource in the cache, then the request doesn’t reach the disk. If we clean the cache, the disk cache will be less useful as the OS will look for the resource on the disk.

Moreover it will also slow the system for a few seconds while the cache is cleaned and every resource required by OS is loaded again in the disk-cache.

Now we will be creating a shell script to auto clear RAM cache daily at 2am via a cron scheduler task. Create a shell script clearcache.sh and add the following lines.

#!/bin/bash
# Note, we are using "echo 3", but it is not recommended in production instead use "echo 1"
echo "echo 3 > /proc/sys/vm/drop_caches"

Set execute permission on the clearcache.sh file.

# chmod 755 clearcache.sh

Now you may call the script whenever you required to clear ram cache.

Now set a cron to clear RAM cache everyday at 2am. Open crontab for editing.

# crontab -e

Append the below line, save and exit to run it at 2am daily.

0  2  *  *  *  /path/to/clearcache.sh

For more details on how to cron a job you may like to check our article on 11 Cron Scheduling Jobs.

Is it good idea to auto clear RAM cache on production server?

No! it is not. Think of a situation when you have scheduled the script to clear ram cache everyday at 2am. Everyday at 2am the script is executed and it flushes your RAM cache. One day for whatsoever reason, may be more than expected users are online on your website and seeking resource from your server.

At the same time scheduled script run and clears everything in cache. Now all the user are fetching data from disk. It will result in server crash and corrupt the database. So clear ram-cache only when required,and known your foot steps, else you are a Cargo Cult System Administrator.

How to Clear Swap Space in Linux?

If you want to clear Swap space, you may like to run the below command.

# swapoff -a && swapon -a

Also you may add above command to a cron script above, after understanding all the associated risk.

Now we will be combining both above commands into one single command to make a proper script to clear RAM Cache and Swap Space.

# echo 3 > /proc/sys/vm/drop_caches && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'

OR

$ su -c "echo 3 >'/proc/sys/vm/drop_caches' && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'" root

After testing both above command, we will run command “free -h” before and after running the script and will check cache.

Clear RAM Cache and Swap Space

That’s all for now, if you liked the article, don’t forget to provide us with your valuable feedback in the comments to let us know, what you think is it a good idea to clear ram cache and buffer in production and Enterprise?

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92 Responses

  1. Nemanja Milosevic says:

    Thank you.

  2. Witold says:

    What is this with echo? echo “echo 3 > /proc/sys/vm/drop_caches“. You should just use echo 3 > /proc/sys/vm/drop_caches, or echo 3 | sudo tee /proc/sys/vm/drop_caches.

    The remark that ‘3’ should not be used in production systems is ridiculous and invalid. It is fully supported and stable for decades. drop_caches is usually only useful when doing benchmarks and timing tests of file systems and block devices, or network attached storage or file systems. You shouldn’t use any drop_cache in the first place in any real production system. It is only for testing and debugging.

  3. Zeki Basbuyuk says:

    It may be necessary to go back to normal operation after this settings. See the following document from kernel.org.

    Returning system back to normal requires user to write “4” into drop_caches

    https://www.kernel.org/doc/Documentation/sysctl/vm.txt

    • Witold says:

      Zeki, no there is no need to write 4. Documentation says it clearly. 4 (bit 2), is only to disable the dmesg / kernel log messages when the drop_caches is issued. writing 1, 2 or 3 is one time thing, and immediately after (or even during) the page cache and other caches will start to be populated back according to system usage.

      I hardly see to ever need to use 4. It might be useful if you do it like every few minutes for some strange reasons, and want to avoid polluting the kernel log (dmesg) by spam of it. But it is just one line, and if you do it sporadically, there is zero reason to use 4.

  4. Steve Sybesma says:

    This is on a personal device that I’m only running one app on that’s not critical but want to make sure the performance is as good as it can be.

    The command to clear the caches doesn’t run until I add sudo. I get access denied without it. Would this be proper?

    sync; echo 3 > sudo /proc/sys/vm/drop_caches

    • Ravi Saive says:

      @Steve,

      Sometimes you need superuser privileges to run commands..Nothing to worry..

      • Steve Sybesma says:

        What I’m asking is WHERE I put the sudo command because it didn’t work in front of sync or echo and I didn’t realize drop-caches was an executable command that would respond to sudo. It seems to work there but I’m just making sure that’s a proper use of sudo in that syntax.

        • Steve Sybesma says:

          Actually found something that seems to work much better than what I last tried:

          sync; sudo sh -c "echo 3 > /proc/sys/vm/drop_caches"

          The buff/cache column on free -h dropped from 340MB to 97MB and the free column went from 78MB to 217MB. Bigger difference than before.

  5. Makler says:

    su -c "echo 3 >'/proc/sys/vm/drop_caches' && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'" root

    Is there a password for this after hitting enter ??? if not why does it ask me?

    and could this be applied for the home desktops too ???

  6. mainmeat says:

    You might want to correct

    swapoff -a && swapon -a
    

    to

    sudo swapoff -a && sudo swapon -a
    
  7. Miphix says:

    When are we going to stop talking about cron, and start talking about systemd timers? It’s time, guys.

    • KPPL says:

      Well, not everybody is in love with systemd.

    • Yoandre Valdes Rodriguez says:

      If you are in love with systemd OK go ahead and use timers but don’t pretend that every body is agree with you.

      • Lucas Ramage says:

        Exactly! I literally went out of my way to comment specifically on this. Why does systemd even need a timer? It’s _supposed_ to be an init system.

    • TW says:

      When are we going to stop talking about systemd and move to openRC? It’s time, guys. Long, long past time in fact.

  8. karthik sailendra says:

    What is the tool you used to generate the Screenshot Gif

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