Improving memory monitoring 🔗 link

On Tue, Apr 14, 2015, at 06:56, Steve Hill wrote:

I'm working on improving my Xymon configuration to reduce the number of false alerts that we get.  In particular, memory monitoring is a bit of a problem so I'm hoping someone will be able to offer some advice.

At the moment, Xymon is set up with something like:

MEMPHYS 100 101
MEMSWAP 20 40
MEMACT 95 97

I pretty much don't care about MEMPHYS.  The problem with MEMSWAP and MEMACT is that they work independently or each other - i.e. the above will give me an alert if > 97% of the RAM is used OR > 40% of swap is
used.

However, this results in warnings for systems that have a lot of idle data in memory.  The Linux kernel will page out idle data (increasing swap usage and reducing RAM usage) and use that space for buffers/caches, and this is a very sensible strategy.  Unfortunately, then Xymon comes along and notices that there's lots of swap in use and throws an alert, even though there's plenty of RAM free.

Basically, I don't care that a machine is 4GB into swap if it has 5GB of free ram - that isn't a problem, it just means there's quite a lot of idle data that the kernel has decided can be paged out.  I do care if it's 4GB into swap and only has 0.5GB of free RAM since this would indicate that it's actually short of memory.

What I really need is to warn if > x% of the RAM is used AND > y% of swap is used - is there a way to do that?

On 2015-04-14 10:00 am, Mike Burger wrote:

On 2015-04-14 7:56 am, Steve Hill wrote:

I'm working on improving my Xymon configuration to reduce the number
of false alerts that we get.  In particular, memory monitoring is a
bit of a problem so I'm hoping someone will be able to offer some
advice.

At the moment, Xymon is set up with something like:

MEMPHYS 100 101
MEMSWAP 20 40
MEMACT 95 97

I pretty much don't care about MEMPHYS.  The problem with MEMSWAP and
MEMACT is that they work independently or each other - i.e. the above
will give me an alert if > 97% of the RAM is used OR > 40% of swap is
used.

However, this results in warnings for systems that have a lot of idle
data in memory.  The Linux kernel will page out idle data (increasing
swap usage and reducing RAM usage) and use that space for
buffers/caches, and this is a very sensible strategy.  Unfortunately,
then Xymon comes along and notices that there's lots of swap in use
and throws an alert, even though there's plenty of RAM free.

Basically, I don't care that a machine is 4GB into swap if it has 5GB
of free ram - that isn't a problem, it just means there's quite a lot
of idle data that the kernel has decided can be paged out.  I do care
if it's 4GB into swap and only has 0.5GB of free RAM since this would
indicate that it's actually short of memory.

What I really need is to warn if > x% of the RAM is used AND > y% of
swap is used - is there a way to do that?

On 14/04/15 15:11, Mike Burger wrote:

I'll say that I've never run into this...I've never had a system swap
memory out to disk unless active memory was utilized at a high
percentage...in either AIX or Linux.

Now, on the other side of this, to take a stab at the question, I'd
wager that, at present, you'd need to script such a test/alert..but I
would agree that it would be useful to be able to set an "alarm if
this or this" or an "alarm if this and this" type scenario. At
present, the only tests I can think of that allow this, "out of the
box" are the process monitors, where you can set minimum and maximum
thresholds.

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  - Steve Hill
    Technical Director
    Opendium Limited     http://www.opendium.com

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On Tue, April 14, 2015 7:56 am, Steve Hill wrote:

On 14/04/15 15:11, Mike Burger wrote:

I'll say that I've never run into this...I've never had a system swap
memory out to disk unless active memory was utilized at a high
percentage...in either AIX or Linux.

It does spontaneously happen from time to time for me - may be the type
of work loads these machines do - they do tend to have a fair amount of
idle data in memory and the kernel quite rightly decides that using that
for caches/buffers would be a better use.

Also, in situations where something _has_ used up a lot of RAM and
therefore pushed stuff out to swap, Xymon continues to warn of high swap
usage after that process has ended because the kernel obviously won't
bother paging stuff back into the newly emptied RAM until it needs to.

Now, on the other side of this, to take a stab at the question, I'd
wager that, at present, you'd need to script such a test/alert..but I
would agree that it would be useful to be able to set an "alarm if
this or this" or an "alarm if this and this" type scenario. At
present, the only tests I can think of that allow this, "out of the
box" are the process monitors, where you can set minimum and maximum
thresholds.

Is there a way of setting analysis.cfg to use a script instead of the
MEM* directives, or would that need to be a completely external job of
some kind?