Knowledge Base

When a physical or virtual machine is idle, its operating system either issues a halt instruction or repeatedly executes an idle loop.

• When an idle virtual machine executes a halt, it stops processing all instructions and requires no cycles on the server's physical CPU. This is the desired behavior in a virtual machine and causes no performance problems.
• When an idle virtual machine executes its idle loop, it is actively executing instructions which run on the underlying physical processor. In this case, performance tools in the guest operating system show an inactive operating system, but the CPU on the server is fully utilized.

Idle loop behavior occurs most often when you try to add virtual processors to a virtual machine. For more information, see the CPU Virtual Machine Configuration section of the vSphere Virtual Machine Administration Guide.

For a virtual machine running a Windows operating system, having the incorrect HAL (Hardware Abstraction Layer) installed makes the guest operating system always spin in its idle loop instead of halting. A virtual machine with 2 or more vCPUs configured should be running with a Multiprocessor or SMP HAL. A virtual machine with 1 vCPU configured should be running with a Uniprocessor HAL.

To determine what HAL your virtual machine is using, and then determine if you are running with the correct vCPU count for the HAL:

1. Right-click the My Computer icon, and choose Properties.
2. Click the Hardware tab.
3. Click the Device Manager tab.
4. Double-click the Computer icon to see which HAL is currently loaded.
5. In the vSphere or VMware Infrastructure client, click on the virtual machine, then click the Summary tab and see how may vCPU's are configured.
   
   If you have 1 vCPU and your virtual machine operating system is configured for a Uniprocessor ACPI, your configuration is correct. If your virtual machine operating system is configured for 2 or more vCPUs and it is running with a Multiprocessor ACPI, your configuration is correct.
   
   Note: Downgrading the HAL from multiprocessor to uniprocessor is not supported by Microsoft, the operating system vendor, and so it is not supported by VMware.

You can add processors to a virtual machine that was created under ESX 2.x or higher, or Workstation 5.5 or higher, but you may need to update the Windows HAL in your guest operating system. This is especially true if you are running a Windows 2000 guest operating system. For more information on adding processors for:

• ESXi 5.x virtual machines, see Configuring Virtual Machines section in the vSphere Virtual Machine Administration.
• ESX/ESXi 4.x virtual machines, see Virtual Machine Hardware Configuration in the Basic System Administation Guide
• ESX 3.x virtual machines, see Changing the Hardware Configuration of a Virtual Machine in the Basic System Administration Guide
• Workstation 7.0 virtual machines, see Use Four-Way Virtual Symmetric Multiprocessing in the Workstation User's Manual
• Workstation 6.5 virtual machines, see Use Two-Way Virtual Symmetric Multiprocessing in the Workstation User's Manual

Note: Even when the correct HAL is installed, some guest operating systems halt more aggressively than others. When the system is otherwise inactive, some guest operating systems may spin in their idle loop for some time before issuing a halt instruction, whereas others may halt immediately. Typically, operating systems that spin in idle loops before halting are running with multiple processors.

For additional information about HAL, see the Microsoft Knowledge Base articles 309283 and 237556.

Note: The preceding links were correct as of April 28, 2010. If you find a link to be broken, provide feedback on the article and a VMware employee will update the article as necessary.

See Also:

• Windows Virtual Machines with Latency-Sensitive I/O Workloads Can Slow Down in Four-Way SMP Configurations (2231).