Does the following apply to you:
vCMP memory provisioning calculations
The amount of memory provisioned to a vCMP guest is calculated using the following formula: (platform_memory- 3 GB) x (cpus_assigned_to_guest/ total_cpus).
As an example, for the B2100 with two guests, provisioned memory calculates as: (16-3) x (2/4) ~= 6.5 GB.
For certain platforms, the vCMP host can allocate a single core to a vCMP guest. However, because a single-core guest has relatively small amounts of CPU resources and allocated memory, F5 supports only the following products or product combinations for a single-core guest:
BIG-IP LTM standalone only
BIG-IP GTM standalone only
BIG-IP LTM and GTM combination only
?
(From 12.1.1 release notes.)