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Full Discussion: RAM Usage discrepancy
Operating Systems OS X (Apple) RAM Usage discrepancy Post 302749963 by jim mcnamara on Sunday 30th of December 2012 09:24:48 AM
Old 12-30-2012
Yup - file caches. All of UNIX - even OSX which loves to carve a unique path - uses memory to cache file data. For example, this is central to the way write() works.

write() does NOT guarantee the data written to a file is immediately part of the physical file on disk. Instead, the kernel parks data in free memory. Periodically this is flushed or "synced" to disk, i.e., dirty cache pages are written to disk.

read() also employs memory this way. Because disk is so much slower to find and return data, while memory is fast, when the filesystem returns data from a big file it parks big chunks of the data in memory. Subsequent reads are satisfied much faster.

You can see this in action. On a quiet system, find a ~100MB file that is human readable.
Code:
time cat bigfile > /dev/null
time cat bigfile > /dev/null

The second cat on quiet system is usually a lot faster than the first. Why? File cache. On hammered systems this may not be true. This is why hammered systems are disproportionately slower than their underused cousins. File cache availability.
 

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CACHEINFO(5)							AFS File Reference						      CACHEINFO(5)

NAME
cacheinfo - Defines configuration parameters for the Cache Manager DESCRIPTION
The cacheinfo file defines configuration parameters for the Cache Manager, which reads the file as it initializes. The file contains a single line of ASCII text and must reside in the /etc/openafs directory. Use a text editor to create it during initial configuration of the client machine; the required format is as follows: <mount>:<cache>:<size> where <mount> Names the local disk directory at which the Cache Manager mounts the AFS namespace. It must exist before the afsd program runs. The conventional value is /afs. Using any other value prevents traversal of pathnames that begin with /afs (such as pathnames to files in foreign cells that do use the conventional name). The -mountdir argument to the afsd command overrides this value. <cache> Names the local disk directory to use as a cache. It must exist before the afsd program runs. The standard value is /usr/vice/cache, but it is acceptable to substitute a directory on a partition with more available space. Although the Cache Manager ignores this field when configuring a memory cache, a value must always appear in it. The -cachedir argument to the afsd command overrides this value. <size> Specifies the cache size as a number of 1-kilobyte blocks. Larger caches generally yield better performance, but a disk cache must not exceed 90% of the space available on the cache partition (85% for AIX systems), and a memory cache must use no more than 25% of available machine memory. The -blocks argument to the afsd command overrides this value. To reset cache size without rebooting on a machine that uses disk caching, use the fs setcachesize command. To display the current size of a disk or memory cache between reboots, use the fs getcacheparms command. EXAMPLES
The following example cacheinfo file mounts the AFS namespace at /afs, establishes a disk cache in the /usr/vice/cache directory, and defines cache size as 50,000 1-kilobyte blocks. /afs:/usr/vice/cache:50000 SEE ALSO
afsd(8), fs_getcacheparms(1), fs_setcachesize(1) COPYRIGHT
IBM Corporation 2000. <http://www.ibm.com/> All Rights Reserved. This documentation is covered by the IBM Public License Version 1.0. It was converted from HTML to POD by software written by Chas Williams and Russ Allbery, based on work by Alf Wachsmann and Elizabeth Cassell. OpenAFS 2012-03-26 CACHEINFO(5)
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