banshee/linux-collection/linux/snmp/memory

Memory of a Linux host, by SNMP

Returns the physical memory, the memory used and available, the buffers and cache, and the share used. Two watches are offered, each adjustable on the probe. Swap is the linux/snmp/swap collector.

Versions

Generic: the memory group of UCD-SNMP-MIB is served by net-snmp on any Linux. net-snmp 5.9 and later add memSysAvail; an older agent is read too, with the fallback below.

How it works

The collector reads the memory group of UCD-SNMP-MIB: memTotalReal, memAvailReal (the free memory), memBuffer, memCached and, when the agent serves it, memSysAvail, in KiB.

Available memory is the kernel's own estimate, MemAvailable, which net-snmp 5.9 and later serve as memSysAvail; the source output then reads memavailable.
An older agent does not serve it: available memory is then free memory plus buffers and cache, which the kernel gives back as soon as an application asks, and source reads free-buffers-cache. Either way, a machine whose cache fills its memory is not saturated.
Used memory is the total minus the available memory.
A total of zero stops the collector as misconfigured: it never shows green on nothing.

Prerequisites

snmpd (net-snmp) running on the host, listening on an address the poller reaches (UDP 161), with the firewall open to the poller only.
The view granted to the poller includes the memory group of UCD-SNMP-MIB (.1.3.6.1.4.1.2021.4).
Read-only access, SNMPv2c or SNMPv3, set on the host or one of its folders (protocol:snmp:* variables).

# /etc/snmp/snmpd.conf
agentaddress udp:161
view   banshee included .1.3.6.1.4.1.2021.4
rocommunity <community> 192.0.2.10 -V banshee
# or: rouser <user> priv -V banshee

Outputs

Name	Label	Unit	Per instance
`total`	Physical memory	MiB	no
`used`	Used memory	MiB	no
`available`	Available memory	MiB	no
`cache`	Buffers and cache	MiB	no
`used-percent`	Used memory	%	no
`source`	Source of available memory: `memavailable` or `free-buffers-cache`	—	no

Watches

Name	Title	Kind	Defaults	Checked by default
`memory-high`	Memory saturated	above a threshold, on `used-percent`	warning above 90 %, critical above 95 %, for 3 runs	yes
`available`	Little memory available	below a threshold, on `available`	critical below 512 MiB; no warning	no

Limits

With the fallback (free-buffers-cache), the part of the cache the kernel cannot give back (shared memory, tmpfs) counts as available: on a machine with a large tmpfs, available memory is overestimated. MemAvailable discounts it.
Total, free, buffers and cache are 31 bits wide in KiB: beyond 2 TiB of memory, the agent itself miscounts them.