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How do you empty the buffers and cache on a Linux system?

Paolo Caparrelli Linux 22 June 2022

If you ever want to empty it you can use this chain of commands.

# free && sync && echo 3 > /proc/sys/vm/drop_caches && free

# free && sync && echo 3 > /proc/sys/vm/drop_caches && free



             total       used       free     shared    buffers     cached

Mem:       1018916     980832      38084          0      46924     355764

-/+ buffers/cache:     578144     440772

Swap:      2064376        128    2064248

             total       used       free     shared    buffers     cached

Mem:       1018916     685008     333908          0        224     108252

-/+ buffers/cache:     576532     442384

Swap:      2064376        128    2064248


You can signal the Linux Kernel to drop various aspects of cached items by changing the numeric argument to the above command.

  • 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
    
    

The above are meant to be run as root. If you're trying to do them using sudo then you'll need to change the syntax slightly to something like these:

$ sudo sh -c 'echo 1 >/proc/sys/vm/drop_caches'

$ sudo sh -c 'echo 2 >/proc/sys/vm/drop_caches'

$ sudo sh -c 'echo 3 >/proc/sys/vm/drop_caches'


NOTE: There's a more esoteric version of the above command if you're into that:

$ echo "echo 1 > /proc/sys/vm/drop_caches" | sudo sh


Why the change in syntax? The /bin/echo program is running as root, because of sudo, but the shell that's redirecting echo's output to the root-only file is still running as you. Your current shell does the redirection before sudo starts.

Seeing what's in the buffers and cache

Take a look at linux-ftools if you'd like to analyze the contents of the buffers & cache. Specifically if you'd like to see what files are currently being cached.

fincore

With this tool you can see what files are being cached within a give directory.

fincore [options] files…



  –pages=false      Do not print pages

  –summarize        When comparing multiple files, print a summary report

  –only-cached      Only print stats for files that are actually in cache.


For example, /var/lib/mysql/blogindex:

root@xxxxxx:/var/lib/mysql/blogindex# fincore –pages=false –summarize –only-cached * 

stats for CLUSTER_LOG_2010_05_21.MYI: file size=93840384 , total pages=22910 , cached pages=1 , cached size=4096, cached perc=0.004365 

stats for CLUSTER_LOG_2010_05_22.MYI: file size=417792 , total pages=102 , cached pages=1 , cached size=4096, cached perc=0.980392 

stats for CLUSTER_LOG_2010_05_23.MYI: file size=826368 , total pages=201 , cached pages=1 , cached size=4096, cached perc=0.497512 

stats for CLUSTER_LOG_2010_05_24.MYI: file size=192512 , total pages=47 , cached pages=1 , cached size=4096, cached perc=2.127660 

stats for CLUSTER_LOG_2010_06_03.MYI: file size=345088 , total pages=84 , cached pages=43 , cached size=176128, cached perc=51.190476 

stats for CLUSTER_LOG_2010_06_04.MYD: file size=1478552 , total pages=360 , cached pages=97 , cached size=397312, cached perc=26.944444 

stats for CLUSTER_LOG_2010_06_04.MYI: file size=205824 , total pages=50 , cached pages=29 , cached size=118784, cached perc=58.000000 

stats for COMMENT_CONTENT_2010_06_03.MYI: file size=100051968 , total pages=24426 , cached pages=10253 , cached size=41996288, cached perc=41.975764 

stats for COMMENT_CONTENT_2010_06_04.MYD: file size=716369644 , total pages=174894 , cached pages=79821 , cached size=326946816, cached perc=45.639645 

stats for COMMENT_CONTENT_2010_06_04.MYI: file size=56832000 , total pages=13875 , cached pages=5365 , cached size=21975040, cached perc=38.666667 

stats for FEED_CONTENT_2010_06_03.MYI: file size=1001518080 , total pages=244511 , cached pages=98975 , cached size=405401600, cached perc=40.478751 

stats for FEED_CONTENT_2010_06_04.MYD: file size=9206385684 , total pages=2247652 , cached pages=1018661 , cached size=4172435456, cached perc=45.321117 

stats for FEED_CONTENT_2010_06_04.MYI: file size=638005248 , total pages=155763 , cached pages=52912 , cached size=216727552, cached perc=33.969556 

stats for FEED_CONTENT_2010_06_04.frm: file size=9840 , total pages=2 , cached pages=3 , cached size=12288, cached perc=150.000000 

stats for PERMALINK_CONTENT_2010_06_03.MYI: file size=1035290624 , total pages=252756 , cached pages=108563 , cached size=444674048, cached perc=42.951700 

stats for PERMALINK_CONTENT_2010_06_04.MYD: file size=55619712720 , total pages=13579031 , cached pages=6590322 , cached size=26993958912, cached perc=48.533080 

stats for PERMALINK_CONTENT_2010_06_04.MYI: file size=659397632 , total pages=160985 , cached pages=54304 , cached size=222429184, cached perc=33.732335 

stats for PERMALINK_CONTENT_2010_06_04.frm: file size=10156 , total pages=2 , cached pages=3 , cached size=12288, cached perc=150.000000 

—

total cached size: 32847278080


With the above output you can see that there are several *.MYD, *.MYI, and *.frm files that are currently being cached.

Swap

If you want to clear out your swap you can use the following commands.

$ free

             total       used       free     shared    buffers     cached

Mem:       7987492    7298164     689328          0      30416     457936

-/+ buffers/cache:    6809812    1177680

Swap:      5963772     609452    5354320


Then use this command to disable swap:

$ swapoff -a


You can confirm that it's now empty:

$ free

             total       used       free     shared    buffers     cached

Mem:       7987492    7777912     209580          0      39332     489864

-/+ buffers/cache:    7248716     738776

Swap:            0          0          0


And to re-enable it:

$ swapon -a


And now reconfirm with free:

$ free

             total       used       free     shared    buffers     cached

Mem:       7987492    7785572     201920          0      41556     491508

-/+ buffers/cache:    7252508     734984

Swap:      5963772          0    5963772


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