9 Replies Latest reply on Jan 18, 2007 8:50 AM by rfunk

    Changing # of CPUs


      I have heard different opinions and would like to query the experts.


      Everyone says to stay with 1 vCPU unless you really need and can truly take advantage of 2, I do understand this.


      But when I migrate my physical boxes to ESX, they start off as a 2-way and I convert them to a 1-way vCPU.


      The question is, whether or not this is the best practice.  I have been told by some that downgrading a Windows OS from ACPI Multiprocessor to ACPI Uniprocessor is not supported by Microsoft or by VMware for that matter.


      I have not had any problems to date - am I destine for issues?


      What are the issues others have had?


      What do others do?

        • 1. Re: Changing # of CPUs
          kucharski Master

          I would always downgrade the CPU within the virtual machine.  But a client I am working at told me it doesn't really matter.  They haven't seen any performance loss if the virtual machine is a dual but only set at 1 cpu.  But again, I would downgrade.  But if you need down the road to change back to a dual, just roll back the driver.



          • 2. Re: Changing # of CPUs
            acr Virtuoso

            Interesting point.. This may be worth a look..



            • 3. Re: Changing # of CPUs
              daniel_uk Expert

              Ive just virtualised a 4 way SAP Server.


              This is sitting at about 30-40% ready time, in VCenter its between 500-1200mhz.


              I think the whole Vcpu thing is kinda now becoming an obvious conclusion....only use them if you have a small amount of VM's on your host.

              • 4. Re: Changing # of CPUs
                acr Virtuoso

                Yeah, but its the impact that all this has on the running OS.. Could do with clarification..

                • 5. Re: Changing # of CPUs

                  Guys please do not hijack my thread and turn it in to a discussion of vSMP or no vSMP.  I (we) should all have a good handle on this.


                  I am looking to see about issues with changing the ACPI from multi to uni when the OS was installed as a multi.


                  I have been told this is unsupported.


                  I want to know what others are doing and if they are having any problems.

                  • 6. Re: Changing # of CPUs
                    sbeaver Guru
                    vExpertUser Moderators

                    The "supported" method is to re-run setup as a repair or upgrade to downgrade the HAL but with that said I have not come into any problems downgrading so far

                    1 person found this helpful
                    • 7. Re: Changing # of CPUs
                      daniel_uk Expert

                      and how would anyone know that you'd done a downgrade?!?!?

                      • 8. Re: Changing # of CPUs
                        acr Virtuoso

                        Not quite what i meant, never mind..

                        • 9. Re: Changing # of CPUs
                          rfunk Enthusiast

                          We have a template we use to create new Windows guest machines. The template is configured to use two CPU’s.  The Windows guests servers which do not need or have a use for two processors, I add the /numproc=1 option in the boot.ini file.


                          This option is a documented Windows option which tells the Windows system to only use one CPU even if there are multiple available.  After I added the /numproc=1 option I change the ESX configuration to reflect one vCPU as well.  I have not had any issues with using these settings.  


                          In the ever changing server world, if you ever would need to active the second processor you remove the /numproc=1 boot.ini command and change it to 2 vCPU on the ESX side and you are good to go.


                          Hope this helps

                          1 person found this helpful