[collectd] memory.c and /proc/meminfo

Florian Forster octo at verplant.org
Thu Nov 17 22:44:41 CET 2005

Hello Filip,

On Thu, Nov 17, 2005 at 02:17:37PM -0600, Hanik, Filip wrote:
> I was getting really funky results, I found that this code worked
> better on Fedora Core 4.

how exactly were those results `funky'? All Linux 2.4/2.6 Boxes I've
seen so far worked perfectly fine..

> ---line 106 memory.c
> 	if (mem_used >= (mem_free + mem_buffered + mem_cached))
> 	{
> 		//mem_used is the total, if mem_used is the biggest number
> 		mem_free = mem_free + mem_buffered + mem_cached;
> 		//mem_used -= mem_free + mem_buffered + mem_cached;
> 		mem_used -= mem_free;
> 		memory_submit (mem_used, mem_buffered, mem_cached, mem_free);
> 	}
> Memory total, is the total of mem_free and mem_used

It's always a bit hard talking about memory utilization, mostly due to
an inconsistent nomenclature. When talking about `used' memory I mean
memory that's actually used by programs. `free' memory is space that's
actually idle, i.e. just sitting there and warming the room.

Given this definition the equation `free + used = total' is NOT true,
since the operating system will use some memory to buffer IO operations.
Under Linux the equation is `free + cached + buffered + used = total',
where `cached' and `buffered' are caches maintained by the kernel. I
won't get too much into Linux memory management, but if you're curious
you'll find many answers in the gentoo `FAQ Linux Memory Management'

Hope this raises the fog a bit ;) Regards,

P.S.: If anyone feels like getting confused, try to understand the
Solaris swap model ;) I hear that those values might be a little
`funky', but the guy telling me wasn't sure either ;)
Florian octo Forster
Hacker in training
GnuPG: 0x91523C3D
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