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VCPGuru
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Run 4vCPUs VMs on an Physical 8 Core ESX host???

I have to run 4 * 4 vCPUs on my ESX Server. I have only ESX Servers with 2 CPUs and each of them has 4 Cores = 8 Cores in total.

Is that enough or what is a Best Practice Value if I have to Run 4vCPUs??

Best Regards Simon Ciglia
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dburgess
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So you will be over committed on CPU by 100%. This is ok if half your vCPU's are doing no real work at any given instant, actually quite common. If they are doing real work then you will have big contention problem and you will see high ready time as you predict. There will be no difference in moving to 2 cpu guests if this is the case as you will still need the number of cores to support the active threads you have (assuming the same throughput).

In the case where the vCPU's are not all busy or they are not busy all together then you should be fine but it does cost ESX some processing to maintain the SMP guest in a valid state - the threads have to progress even if they are not 'busy'. If ESX is working hard to acheive this then you will see %CSTP in esxtop rise. This roughly means that ESX is having to stop/reschedule threads in the VM just to keep them active when it has real work it would otherwise be doing. So at this point you may better of with smaller VM's. Checkout our scheduling paper on the VROOM blog.

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weinstein5
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It is enough - why do you need the 4 vCPUs? Do you think you can by with one? - running the 4 4xvCPU VMs, depending on their kiad, you might run into contention issues -

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VCPGuru
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Should i use instead of that more server with 2 vCPUs? Could I find out, if i have an contention issue, if I look at the CPU ready value (i think a value over 5% is bad)?

Thank you in advance!

Best Regards Simon Ciglia
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dburgess
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So you will be over committed on CPU by 100%. This is ok if half your vCPU's are doing no real work at any given instant, actually quite common. If they are doing real work then you will have big contention problem and you will see high ready time as you predict. There will be no difference in moving to 2 cpu guests if this is the case as you will still need the number of cores to support the active threads you have (assuming the same throughput).

In the case where the vCPU's are not all busy or they are not busy all together then you should be fine but it does cost ESX some processing to maintain the SMP guest in a valid state - the threads have to progress even if they are not 'busy'. If ESX is working hard to acheive this then you will see %CSTP in esxtop rise. This roughly means that ESX is having to stop/reschedule threads in the VM just to keep them active when it has real work it would otherwise be doing. So at this point you may better of with smaller VM's. Checkout our scheduling paper on the VROOM blog.

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VCPGuru
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could you please post the link to the VROOM document?

thx

Best Regards Simon Ciglia
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Rumple
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best practice is to ignore everything you thought you knew about the physical world and start out with every VM running with a single CPU and as little ram as possible...if you are seeing issues with performance but the host is not busy, then add a CPU...

Unless absolutely necessary avoid having many SMP systems as the scheduler has to remove 2 single CPU's from working in order to give the SMP VM cpu time...do that too many times and you will see high %ready times and you will have scheduling issues.

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