[collectd] memory.c and /proc/meminfo

[Florian Forster]

Hi again :)

On Thu, Nov 17, 2005 at 03:53:02PM -0600, Hanik, Filip wrote:
> On fedora 4, you get the following output
> 
> [root at filip sbin]# free -m
>              total       used       free     shared    buffers     cached
> Mem:          2026       1969         56          0         43       1270
> -/+ buffers/cache:        655       1370
> Swap:         1983          0       1983
> 
> [root at filip sbin]# cat /proc/meminfo
> MemTotal:      2074728 kB
> MemFree:         57524 kB
> Buffers:         44148 kB
> Cached:        1301648 kB
> 
> So as you can see, the buffers and cached are not necessarily part of
> the memory that is "used".  To get the true usage from the stats (ie
> percentage free RAM) I had to do the modification.

Okay, this is what collectd currently does:
    MemTotal == MemFree + Buffers + Cached + MemUsed
=>  MemUsed  := MemTotal - (MemFree + Buffers + Cached)

In numbers:
    MemUsed  := 2074728 - (57524 + 44148 + 1301648) == 671408

    MemFree  =~  2.8%
    Buffers  =~  2.1%
    Cached   =~ 62.7%
    MemUsed  =~ 32.4%

On Thu, Nov 17, 2005 at 02:17:37PM -0600, Hanik, Filip wrote:
> 	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);
> 	}

What you do here is:
    MyFree = MemFree + Buffers + Cached
    MyUsed = MemTotal - MyFree

In numbers:
    MyFree   := 57524 + 44148 + 1301648 == 1403320
    MyUsed   := 2074728 - 1403320 == 671408

    MyFree   =~ 67.6%
    MyUsed   =~ 32.4%
    Buffers  =~  2.1%
    Cached   =~ 62.7%

Clearly, for these numbers to make sense you need to set `Buffers' and
`Cached' to zero.

> Before we made this modification, we would have numbers like 97%
> memory used, cause collectd didn't take into consideration that
> cached/buffered memory might be available for us.

collectd uses four numbers. If you add MemUsed, Buffers and Cached
you'll get a very large number that is far from practical (this is the
kind of information e.g. Net-SNMPd will give you). 
Adding MemFree, Buffers and Cached will give you `anything not used bu
programs' which may be interesting, e.g. for monitoring purposes..

I guess my point is: All four numbers are available right now. To get
percentages you need to get (at least) two numbers and do some
computation, so adding MemFree, Buffers and Cached is applicable.. So I
don't see why you'd need to change anything within collectd..

> We are using collectd to report back the stats to nagios, so a few
> bytes here and there wont make a difference, But we need to report
> correcly on memory utilization (ie, memory taken up by apps) in case a
> process starts leaking.

Interesting :) How do you feed the data to nagios? You're not using the
RRD files, are you?!

Regards,
-octo
-- 
Florian octo Forster
Hacker in training
GnuPG: 0x91523C3D
http://verplant.org/
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