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Performance with 1xvCPU and 2xvCPU VMs

Hi. I've been reading some info re. running single vCPu and dual/quad vCPU virtual machines on the same ESX host.

I believe that not only can assigning multiple vCPUs to a VM that doesn't really need it can affect performance of other VMs by using CPU cycles that could be used elsewhere, but I've also read that the CPU scheduler in ESX has to work overtime to work out what gets scheduled next when you start mixing VMs with different numbers of vCPUs.

Is this correct, and how much effect does this have in real environment?

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SecretWeapon

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paulo999

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Thanks for the comment, SecretWeapon.

I have 160 physical servers that we want to virtualise, ranging from 1 to 4 cores (some are old servers) so I'm really thinking of this from a bigger scale.

If I had say, a DL380 G5 with 2xQuad Core Xeons and 32GB of RAM, running 16 VMs, or maybe a DL580 G5 with say 4xQuad Core Xeons, 64GB of RAM and running 32 VMs, how much would running a mixture of single, dual and quad vCPU machines on the same host affect performance?

If it would affect performance a lot would I be better off splitting my ESX clusters into single, dual and quad core VM clusters? Or maybe look at trying to configure DRS to only allow VMs with the same number of vCPUs to run on the same host (I don't know if the later is actually possible?). Thanks.

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Paul Solovyovsky

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paulo999

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Thanks for that. This is the current method I use - just use one vCPU and only use multiple if there is a specific reason to do so.

Regarding your comments about changing the Windows HAL; I remember on NT 4.0 if you went from one CPU to multiple CPUs you had to manually change the kernel, however with Win 2003 server I've just added the vCPUs and its registered them - am I missing something?

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Paul Solovyovsky

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EFHC

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paulo999

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Well my question was really how much stress is put in the VMWare scheduler when mixing single vCPU and multi vCPU VMs on the same host. But this info is good just the same. Thanks.