Virtual Processor utilization

[Viperman]

A consulting customer asked me the question of why his virtual machine was running so slow and I explained that he needed to reduce the number of processors in the VM instead of increase it.  This is a direct quote from the customer.

"Just wanted to thank you for suggesting we change one of our screaming servers from 4 procs to 1. 
The utilization went from 95% to 6%!

Awesome!"

The reason for this is that in this case his physical machine had dual quad core processors which mean a total of 8 cores available.  The virtual machine he was using had 4 vCPU's in it.  The way that VMware ESX works is that all vCPU's must be scheduled at the same time or none of them will run. 

Since he had numerous virtual machines running on this physical box they were all contending for processor utilization from the VMkernel.  Since all 4 vCPU's had to be scheduled at the same time there was alot of waiting on the part of the VM, this could be seen by viewing the CPU Ready values of the Virtual Machine from the VIC.

By reducing the number of CPU's in the Virtual machine it could pretty much be scheduled on any core at any time.  My suggestion is usually to run a single vCPU to start and then increase the number of vCPU's as needed.  Remember that some older OS's could have a problem with a single processor HAL so you might need to upgrade the HAL so that it would allow the use of more processors.

[pizzoja]

Do you think this would be an issue if you had only 1 virtual machine running on the sytem?  They way i understand it is that if I only had 1 virtual machine running on the system then there would be nothing else competing for the processing time with the exception of the ESX server itself.

Joel

[Ekoesling]

How do you see the CPU ready values within the VIC?

Thanks

[pizzoja]

i am not sure what you mean.  I have not actually built this yet.  i am still in the design phase of the project.

[Viperman]

Do you think this would be an issue if you had only 1 virtual machine running on the sytem?  They way i understand it is that if I only had 1 virtual machine running on the system then there would be nothing else competing for the processing time with the exception of the ESX server itself.

Joel

It shouldn't be, but how many physical procs do you have?  The only thing competing for the proc would be the Service console.

[Viperman]

How do you see the CPU ready values within the VIC?

Thanks

You need to go to the performance tab, and then change the CPU to real time monitoring and choose the CPU counter of CPU ready.  CPU ready shows you how long that a CPU must wait prior to being able to use the available physical CPUs

[pizzoja]

I have 4 procs that will have 1 VM,  I know not the most effecient use of VM software to it is being done for the abillity to quickly cut over to more powerful hardware.

Thanks, for all you help. 

[Viperman]

Even on this setup I would still start with a single VCPU.  You can always increase the VCPUs if you need to, but most of the time people don't need to.  You should have more then plenty of procs though.