Message-ID: <445B666E.6000908@modgraph-usa.com>
Date: Fri, 05 May 2006 07:51:26 -0700
From: "Craig A. James" <cjames@modgraph-usa.com>
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To: "mcelroy, tim" <tim.mcelroy@bostonstock.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Memory and/or cache issues?
References: <0C4841B42F87D51195BD00B0D020F5CB044B2611@MORPHEUS>
In-Reply-To: <0C4841B42F87D51195BD00B0D020F5CB044B2611@MORPHEUS>
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mcelroy, tim wrote:
> Sorry, been up all night and maybe provided too much information or not 
> the right information and only confused folks, tired I guess.  When I 
> say 'in use' I am referring to the 'used' column.  Thanks all who have 
> responded to this inquiry, I appreciate it.
> 
> Here's free from PROD001:
> [root@wbibsngwyprod001 kernel]# free -k -t
>              total       used       free     shared    buffers     cached
> Mem:       7643536    6975772     667764          0     165496    5393396
> -/+ buffers/cache:    1416880    6226656
> Swap:      8185108       5208    8179900
> Total:    15828644    6980980    8847664

On Linux (unlike most Unix systems), "used" includes both processes AND the kernel's file-system buffers, which means "used" will almost always be close to 100%.  Starting with a freshly-booted system, you can issue almost any command that scans files, and "used" will go up and STAY at nearly 100% of memory.  For example, reboot and try "tar cf - / >/dev/null" and you'll see the same sort of "used" numbers.

In My Humble Opinion, this is a mistake in Linux.  This confuses just about everyone the first time they see it (including me), because the file-system buffers are dynamic and will be relenquished by the kernel if another process needs memory.  On Unix systems, "used" means, "someone else is using it and you can't have it", which is what most of us really want to know.

Craig