[hobbit] Memory check

[Henrik Stoerner]

On Tue, Feb 07, 2006 at 07:41:23PM -0500, David Gilmore wrote:
> My hobbit server (Fedora FC4) has 1.25 gig of memory installed.  When the
> server is backed, up using Retrospect client, REAL memory usage spikes from
> 34% to 97% and stays at that level until a reboot.  When I check the system
> performance, using the built in system monitor, user memory is at 18.9%.
> Dell Open Manage is using the most memory at 3% with a few additional
> processes between 1% and 2%.  Everything else is well under 1%.  What
> exactly is hobbit reporting on when it says that Physical/Real memory is at
> 97%, Actual memory is at 17%, and Swap is at 0%?

Hobbit reports the output from the "free" command. It probably looks
somewhat like this after you've run a backup:

             total       used       free     shared    buffers cached
Mem:        646432     642172       4260          0     167676 136068
-/+ buffers/cache:     338428     308004
Swap:       511992          4     511988

The "Mem" line here tells you that there is 640 MB RAM installed, and
all except 4 MB is being "used". However, a lot of that is used for
"buffers" and "cache", which is the Linux kernel's dynamically resized
disk cache; if an application needs more RAM that is "free", the
disk cache/buffers are discarded and the memory made available to the 
application.

So that's why the "-/+ buffers/cache" line is interesting: This shows 
the used/free memory count if the buffers/cached is counted as "free"
memory. Hobbit report this as the "actual" memory count.

So a Linux system will practically always have a REAL memory usage
close to 100% (Linus Torvalds once said that "unused RAM is *wasted*
RAM, and there's no reason to spend lots of money on something that
isn't used" - quoting from memory). The ACTUAL memory usage (should)
be a lot less, and is what you'll want to keep an eye on.


Regards,
Henrik