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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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vzcalc(8)							    Containers								 vzcalc(8)

NAME
vzcalc - calculate resource usage of a container SYNOPSIS
vzcalc [-v] CTID DESCRIPTION
This utility displays the share of the host system resources a particular container is using. If the container is running, the current usage is displayed. High utilization values (>100%) mean the system is overloaded (or the container has an invalid configuration). Current Shows the amount of the resources consumed by the container at a given time. Promised Shows the resources soft limit values "promised" for a given container. Max Shows the resources hard limit values "promised" for a given container. If the -v option is specified, the following additional information is also displayed: Low Mem The part of memory residing at lower addresses and directly accessed by the kernel (only makes sense for 32-bit architectures). Total RAM Total memory. Mem+Swap Amount of memory available for applications (both RAM and swap space). Alloc Mem Standard memory allocations made for applications in a container. This is a more "virtual" system resource than RAM or RAM and swap. Num. Proc Number of processes. OPTIONS
-v Display additional information. EXIT STATUS
Normally, the exit status is 0. On error, the exit status is 1. LICENSE
Copyright (C) 2000-2009, Parallels, Inc. Licensed under GNU GPL. OpenVZ 10 Dec 2009 vzcalc(8)
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