Understanding Default SNMP Utilization Calculations

Understanding Default SNMP Utilization Calculations

Problem

You have questions about how ktranslate calculates various utilization metrics.

Background

ktranslate returns raw SNMP polling data in almost all cases. The following metrics are exceptions where calculations or conversions are applied:

• CPU utilization %
• Memory utilization %
• Interface utilization %
• Interface error %
• Metrics with enum or conversion functions applied in their profile configuration

Solution

CPU Utilization %

Metric name: kentik.snmp.CPU

CPU is generally returned by a direct OID that provides an integer or float value representing percentage utilization. In rare cases where only a CPU idle value is available, ktranslate converts it using:

CPU = 100 - CPU Idle

Memory Utilization %

Metric name: kentik.snmp.MemoryUtilization

Unlike CPU, memory utilization is rarely available as a direct OID. ktranslate calculates it based on whichever combination of OIDs is available:

If Memory Used and Memory Free are available:
  Memory Utilization = ( Memory Used / (Memory Free + Memory Used) ) * 100

If Memory Total and Memory Free are available:
  Memory Utilization = ( (Memory Total - Memory Free) / Memory Total ) * 100

If Memory Total and Memory Used are available:
  Memory Utilization = ( Memory Used / Memory Total ) * 100

If Memory Total, Memory Buffer, and Memory Cache are available:
  Memory Utilization = ( ( Memory Total - (Memory Buffer + Memory Cache) ) / Memory Total ) * 100

Interface Utilization %

Metric names: kentik.snmp.IfInUtilization | kentik.snmp.IfOutUtilization

Interface utilization follows the industry standard approach: calculate the delta in bits and divide by the product of the interface's configured speed and the polling time delta.

Assuming 1 is the previous data point and 2 is the most recent:

( ( ifHCInOctets_2 - ifHCInOctets_1 ) * 8 * 100 ) / ( (sysUptime_2 - sysUptime_1) * ifSpeed )

Numerator:

• Take the latest octet count minus the previous sample (delta).
• Multiply by 8 to convert octets to bits.
• Multiply by 100 to set up percentage calculation.

Denominator:

• Take the latest sysUptime minus the previous sample (polling interval delta).
• Multiply by the configured ifSpeed (reported in bits).

Divide numerator by denominator to get the utilization percentage.

ktranslate uses ifHCInOctets (receive) or ifHCOutOctets (transmit), substituting ifHighSpeed for ifSpeed in the denominator as needed. See the source code for details.

Tip: A common cause of inaccurate interface utilization percentages is a misconfigured interface speed on the device that doesn't reflect the actual link speed. For example, a 1 Gbps MPLS circuit on a 10 Gbps interface would show utilization at only 10% of the real value. Consult your vendor's documentation for how to set the correct interface bandwidth.

Interface Throughput

Metric names: kentik.snmp.ifHCInOctets | kentik.snmp.ifHCOutOctets

Interface throughput uses ifHCInOctets (receive) or ifHCOutOctets (transmit).

Key unit conversions:

• 1 octet = 1 byte
• 1 octet = 8 bits

The raw SNMP measurement counts octets (bytes). To get bytes, use the metric directly. To get bits per second, multiply by 8 and convert to a per-second rate.

Interface Errors %

Metric names: kentik.snmp.ifInErrorPercent | kentik.snmp.ifOutErrorPercent

Interface error percentage is calculated using ifInErrors or ifOutErrors divided by ifHCInUcastPkts or ifHCOutUcastPkts. See the source code:

( ifInErrors / ifHCInUcastPkts ) * 100
or
( ifOutErrors / ifHCOutUcastPkts ) * 100

SNMP conversions

Other SNMP metrics are converted based on the enum and conversion functions in their respective SNMP profile.

Profile Setting	Usage
tag	Overrides the name attribute with a friendly name sent in the exported payload.
enum:[]	Converts the integer value of a dimensional metric into an enumerated string value — for example, converting kentik.snmp.if_AdminStatus to up, down, or testing.
conversion: hextoint: <current>: <desired>	Converts hexadecimal values to integer format. Options for current: LittleEndian | BigEndian. Options for desired: uint16 | uint32 | uint64.
conversion: hextoip	Converts hexadecimal values to 4-octet IPv4 strings.
conversion: hwaddr	Converts hexadecimal values to MAC address strings.
conversion: powerset_status	Enumerates the upsBasicStateOutputState ASCII string in the POWERNET-MIB.
conversion: regexp	Captures a substring from the OID output via regex match. Must be quoted with backslashes escaped. Example: input " 5 Secs ( 96.3762%) 60 Secs ( 62.8549%) 300 Secs ( 25.2877%)" with conversion "regexp:60 Secs.*?(\\d+)" returns 62.
conversion: to_one	Creates a gauge metric with value 1 for non-numeric scalar OIDs that have no enumeration options, such as tlUpsTestResultsDetail which returns a DisplayString.