Virtual machines configured with CPU limits running on ESX/ESXi 4.x hosts experience performance degradation

Products

• VMware vSphere ESXi

Issue/Introduction

Symptoms:

• The virtual machine configured with CPU limit experiences a drop in performance on ESX/ESXi 4.x hosts when the CPU limit is reached (%MLMTD greater than 0)
• This performance degradation does not occur on ESX/ESXi 3.5 hosts
• The network latency of the virtual machine is below the acceptable level
• Ping round trip times are as high as hundreds milliseconds, but returns to normal values when the CPU limit on the virtual machine is removed

Note: If you have a defined a CPU limit and you want to check if the virtual machine is currently throttled to reflect that limit, check the %MLMTD value for the virtual machine in esxtop's CPU view. For more information on using esxtop, see Interpreting esxtop Statistics.

Environment

VMware ESX 4.1.x
VMware ESXi 4.0.x Embedded
VMware ESXi 4.1.x Installable
VMware ESXi 4.1.x Embedded
VMware ESXi 4.0.x Installable
VMware ESX 4.0.x

Resolution

This issue is resolved in vSphere 4.0 Update 3.

To workaround this issue, check if the virtual machine requires a CPU limit. If not, remove the CPU limit.

 

If you do not want to remove the CPU limit, reduce the scheduling quantum.

 

To reduce the scheduling quantum:

1. Log in to the ESX/ESXi host using the vSphere Client.
2. Select the host and click the Configuration tab.
3. Click Advanced Settings and click the Cpu page.
4. Set the value of Cpu.Quantum to 5.
   
   Note: The default value of Cpu.Quantum is 50.
    
5. Reboot the host for the changes to take effect.

Additional Information

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