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Multi-Core setting for VMWare VM's

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Cali3350

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Mar 22, 2002
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Setting up some VM's for my school's CS department. All Machines run Q6600's at stock (4 core Core 2 duo based). They want Ubuntu for students and Redhat for Professors. As I am configuring all of this I couldnt help notice VMWare allows multi-core in its preferences. Does this need to be enables? Will it see a performance difference?

I did try googling however just came across licensing issues.
 
what vmware product are you using to host this? esx/esxi or workstation/server/player? general rule of thumb is use 1 vCPU on any given VM in all cases, unless there is a very specific reason not to do so (a legitimate one, that is... not just because you want them to have a "dual core"). the way that the calls from the guests are handled by the host favors single core guests more than multi-core ones (in short).
 
As sabregen said, 99% of the time you should use 1 vCPU. The one noted exception is SQL. Others are multithreaded apps that absolutely require more than one cpu. You can test for yourself to be certain, but everytime this issue has come up the answer has always been 1 vCPU. A few of us have even benchmarked the difference and there was a significant speed penalty by using more than one (for typical VMs). VMware schedules threads differently for the guest and as a result it is usually most efficient to use one vCPU.
 
you should really only use one core?

i have been running server 2008 for school with 2 cores in vm. would i get better performance if i changed it to 1 core?
 
tricky0502 said:
you should really only use one core?

i have been running server 2008 for school with 2 cores in vm. would i get better performance if i changed it to 1 core?
Click to expand...

What are you running on the server 08? Active Directory? DHCP? SQL?
 
well it is for school

it is running dhcp print ad (as of now). im not sure what other rolls will be added.
 
It is VMWare Workstation. The major thing for school is the students just need a *nix development environment for some schoolwork and the professors want it for some research. I just set up each with 1 core as you suggested, none of them should need SMP for schoolwork / research, I just thought it might be faster. Thanks a lot guys. Very much appreciated!
 
You also need to consider vCPU scheduling when you have multiple vCPUs and multiple VMs. The more vCPUs you have configured, the harder it is to schedule the VM to execute because it needs exclusive access to x physical cores (where x = number of vCPUs). With ESX you can monitor this overhead with the CPU Ready metric, which measures the time in milliseconds that the VM was ready to execute but had to wait for CPU resources. This number can go from 10ms for 1 vCPU to 150ms+ for 4vCPU. If your VM doesn't NEED more than one vCPU (and most do not), then the performance hit isn't worth it.
 
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