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Top Forums UNIX for Advanced & Expert Users echo 1 > /proc/sys/vm/drop_caches a good idea? Post 302462902 by Neo on Friday 15th of October 2010 09:24:06 AM
Old 10-15-2010
Hi Eric,

Actually, it really does not matter much, frankly speaking. I've experimented with dropping caches in Linux so many times on our production web server (LAMP).

Linux does such a great job at using all available memory, and reclaiming it when needed, so it is better to let Linux manage those things.

When you drop the cache (or caches), you will see the CPU load go up (sometimes way up) because the cache is gone. Available RAM goes up, but it does not matter because performance is slower because the cache is empty.

Then over time, if you leave the caches off, the performance will suffer because you are not taking advantage of the cache.

Linux tries to use all available RAM, so the caches will fill over time if you don't instruct Linux to drop the caches, and this is a good thing. You will see available RAM go down, but don't worry, it is available for applications when needed because applications take higher priority than cache.

You want Linux to use all the RAM. That is a good thing because the kernel is basically using all available RAM that is not used by applications (and the OS) for cache. Dropping caches has little positive effect on performance. In fact, it tends to have a negative effect. The reason is that you are not really making more RAM available to the apps, because the apps already have been given the RAM needed. You simply are dropping the cache, which degrades performance.

Regarding, swappiness, the same is basically true. Linux will dump the cache before swapping, as I recall, but I would need to read up on that again to see exactly how it works.

We have experimented with swappiness, and ours is currently set to:
Code:
# cat swappiness
20

... and FYI:

Code:
# cat drop_caches
0

This User Gave Thanks to Neo For This Post:
 

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KDESTROY(1)							   MIT Kerberos 						       KDESTROY(1)

NAME
kdestroy - destroy Kerberos tickets SYNOPSIS
kdestroy [-A] [-q] [-c cache_name] DESCRIPTION
The kdestroy utility destroys the user's active Kerberos authorization tickets by overwriting and deleting the credentials cache that con- tains them. If the credentials cache is not specified, the default credentials cache is destroyed. OPTIONS
-A Destroys all caches in the collection, if a cache collection is available. -q Run quietly. Normally kdestroy beeps if it fails to destroy the user's tickets. The -q flag suppresses this behavior. -c cache_name Use cache_name as the credentials (ticket) cache name and location; if this option is not used, the default cache name and location are used. The default credentials cache may vary between systems. If the KRB5CCNAME environment variable is set, its value is used to name the default ticket cache. NOTE
Most installations recommend that you place the kdestroy command in your .logout file, so that your tickets are destroyed automatically when you log out. ENVIRONMENT
kdestroy uses the following environment variable: KRB5CCNAME Location of the default Kerberos 5 credentials (ticket) cache, in the form type:residual. If no type prefix is present, the FILE type is assumed. The type of the default cache may determine the availability of a cache collection; for instance, a default cache of type DIR causes caches within the directory to be present in the collection. FILES
FILE:/tmp/krb5cc_%{uid} Default location of Kerberos 5 credentials cache SEE ALSO
kinit(1), klist(1) AUTHOR
MIT COPYRIGHT
1985-2013, MIT 1.11.3 KDESTROY(1)
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