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Determining if multiple virtual CPUs are causing performance issues (1005362)

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Symptoms

You may experience these performance issues with a multiple CPU virtual machine running on an ESX host:

  • Poor transfer speeds when copying data to or from a virtual machine
  • Backup jobs time out or are very slow
  • A virtual machine performs poorly

Purpose

This article helps you determine if multiple virtual CPUs (vCPUs) are causing performance issues.

Resolution

To determine if multiple vCPUs assigned to your virtual machine is causing poor performance:
  1. Open a console prompt on the ESX host or initiate an SSH connection to it. For more information, see Opening a command or shell prompt (1003892).
  2. Type esxtop and press Enter.
  3. In the CPU screen, check the %CSTP value. If this number is higher than 3.00, the performance issues may be caused by the vCPU count. Try lowering the vCPU count of the virtual machine by 1.

    Note: The %CSTP value represents the amount of time a virtual machine with multiple virtual CPUs is waiting to be scheduled on multiple cores on the physical host. The higher the value, the longer it waits and the worse its performance.  Lowering the number of vCPUs reduces the scheduling wait time.
To lower the vCPU count:

Note: The virtual machine must be powered off to perform these steps.
  1. Right-click on the virtual machine and click Edit Settings.
  2. Click CPUs.
  3. Use the Number of virtual processor drop-down to lower the vCPU count by 1.
  4. Click OK
  5. If your virtual machine still experiences performance issues, and if its kernel or HAL can handle switching to a single vCPU, lower the vCPU count to 1.

    Warning: If your virtual machine's kernel or HAL cannot handle switching to a single vCPU, unexpected behaviour may occur.

See Also

Update History

04/01/2013 - Added ESXi 5.1.x and vCenter Server 5.1.x to Products 10/07/2013 - Added ESXi 5.5.x, vCenter Server 5.5.x and the vCenter Server Appliance 5.0, 5.1 and 5.5 to Products

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