echo 1 > /proc/sys/vm/drop_caches a good idea? Post: 302462902

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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

10-15-2010

Administrator

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

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LEARN ABOUT FREEBSD

linprocfs

LINPROCFS(5)						      BSD File Formats Manual						      LINPROCFS(5)

NAME

     linprocfs -- Linux process file system

SYNOPSIS

     linproc	     /compat/linux/proc      linprocfs	     rw 0 0

DESCRIPTION

     The Linux process file system, or linprocfs, emulates a subset of Linux' process file system and is required for the complete operation of
     some Linux binaries.

     The linprocfs provides a two-level view of process space.	At the highest level, processes themselves are named, according to their process
     ids in decimal, with no leading zeros.  There is also a special node called self which always refers to the process making the lookup
     request.

     Each node is a directory containing several files:

     exe     A reference to the vnode from which the process text was read.  This can be used to gain access to the process' symbol table, or to
	     start another copy of the process.

     mem     The complete virtual memory image of the process.	Only those addresses which exist in the process can be accessed.  Reads and writes
	     to this file modify the process.  Writes to the text segment remain private to the process.

     Each node is owned by the process's user, and belongs to that user's primary group, except for the mem node, which belongs to the kmem group.

FILES

     /compat/linux/proc 	      The normal mount point for the linprocfs.
     /compat/linux/proc/cpuinfo       CPU vendor and model information in human-readable form.
     /compat/linux/proc/meminfo       System memory information in human-readable form.
     /compat/linux/proc/pid	      A directory containing process information for process pid.
     /compat/linux/proc/self	      A directory containing process information for the current process.
     /compat/linux/proc/self/exe      The executable image for the current process.
     /compat/linux/proc/self/mem      The complete virtual address space of the current process.

EXAMPLES

     To mount a linprocfs file system on /compat/linux/proc:

	   mount -t linprocfs linproc /compat/linux/proc

SEE ALSO

     mount(2), unmount(2), procfs(5), pseudofs(9)

HISTORY

     The linprocfs first appeared in FreeBSD 4.0.

AUTHORS

     The linprocfs was derived from procfs by Pierre Beyssac.  This manual page was written by Dag-Erling Smorgrav, based on the procfs(5) manual
     page by Garrett Wollman.

BSD
								  August 10, 1994							       BSD