Ticket 12 – CMP Demotion Caused by iRule Globals

Title: “Why is this virtual server only using one CPU?”

Ticket Description

Operations has reported that the virtual server web-app-https on WestRegion-bigip-01 appears to be using only a single TMM/CPU, even though the device has multiple TMM instances available.

This behavior is impacting throughput and scalability. You have been asked to determine why Clustered Multiprocessing(CMP) is demoted on this virtual server and which configuration element is responsible.

For information on CMP right-click this link and open in new tab: https://my.f5.com/manage/s/article/K14358

Context

Device Name: WestRegion-bigip-01

Virtual Server: web-app-https

Observation: CMP is demoted for this virtual server, so traffic is processed by a single TMM/CPU rather than being distributed across available TMM instances.

Tasks

Use the AI Assistant and enter the following prompt:

Show full configuration details for the web-app-https virtual server on WestRegion-bigip-01, including assigned iRules and why CMP may be in a demoted state.

From the returned information and the TMUI on WestRegion-bigip-01:

• Identify which iRule(s) are attached.
• Inspect the iRule code and locate any regular global variables using the ::name syntax (for example, set ::request_count 0).

Explain why this example variable (::variable) in an iRule causes CMP to be demoted:

• Regular global variables are not shared correctly across TMM instances.
• To maintain consistency, BIG-IP forces the virtual server to run on a single TMM, reducing parallelism.

As part of your explanation, consider:

• Regular global variables (::name) are not shared across TMM instances.
• CMP demotion reduces a virtual server to a single TMM/CPU.
• Mixing CMP-compatible and CMP-incompatible iRules on the same virtual server still results in demotion.

Describe how you would refactor the iRule to keep it CMP-safe, for example:

• Replace non-static globals with per-connection variables.
• Use static:: variables only for read-only configuration data.
• Avoid constructs documented as CMP-incompatible.

Deliverables

Briefly answer the following:

• What is the issue causing CMP demotion according to the AI agent in Insight?

Review the following for additional information and trends:

• The CMP state of web-app-https and how you verified it
• Which iRule (and which specific lines) are responsible for CMP demotion
• How you would rewrite the iRule to be CMP-compatible while preserving its intended behavior

Hints

• Even a single ::variable usage is enough to demote CMP.
• Attaching both CMP-safe and CMP-unsafe iRules to the same virtual server does not “average out” — the CMP-unsafe behavior still causes demotion.
• Compare CMP mode and TMM distribution before and after removing the CMP-unsafe iRule.

This concludes Ticket 12.

Go to Ticket 13 - WAF Blocking a Known Apache Struts-Like Probe

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