Windows guest operating system exhibits performance issues due to high memory usage by the PFN database

Products

• VMware vSphere ESXi

Issue/Introduction

Symptoms:

• Windows Task Manager reports high CPU utilization
• Windows guest operating systems exhibit performance issues due to high memory usage by PFN database
• You do not notice any issue in esxtop and all parameters are reported to be normal
• Only virtual machines with hot add memory are affected

Cause

When hot memory is enabled, the guest operating system uses a huge amount of kernel memory space in the PFN database. Windows operating system does not have dynamic PFN allocation. When adding memory to the virtual machine, to make it visible to the guest operating system, the PFN database needs to be dynamic as Windows lacks this feature.

VMware has set a maximum value for hot add memory. By default, this value is 16 times the amount of memory assigned to the virtual machine. For example, if the virtual machine memory is 2 GB, the maximum value for hot add memory is 32GB (2x16).

Resolution

If you want to use hot add memory, but do not want to grow the memory size by 16 times, you can set the advanced configuration option memory.maxgrow to any value between 2 and less than 16.

To set this parameter:

1. Power off the virtual machine.
2. Right-click the virtual machine and click Edit Settings.
3. Click Options > General > Configuration Parameters.
4. Click Add Row.
5. Under Name, type memory.maxgrow in the Value field and enter any value between 2 and 16.
   
   For example, if this value is set it to 2, a virtual machine with 8GB of memory can grow up to 16GB without powering off the virtual machine.