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VMWare, number of vCPU

Posted on 2012-03-17
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Last Modified: 2012-03-18
I know I am a little slow about this, but I have been experimenting, and yes, my VM's perform better with less vCPU, instead of more, especially when they are not running at a high CPU utilization

So from reading the posts and the documentation, what I am gleaning is that the scheduler is swapping the vCPU'sfrequently, and with less vCPU it basically swaps less often,a nd can obtain a higher performance.

Is this correct.  Can anyone enlighten me as to the 'core' reasons less vCPU seems to improve performance.
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Question by:Los Angeles1
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Andrew Hancock (VMware vExpert / EE MVE^2) earned 500 total points
ID: 37733774
You server applications need to take advantage of vSMP.

How are you testing, are you using Exchange, SQL that can take advantagebof multiprocessors?

Your findings are correct. This is how vSMP works.

Only allocate more CPUs if your servers applications can take advantage.
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LVL 119
ID: 37733775
less time spent waiting for cpu states.
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Expert Comment

by:Aaron Tomosky
ID: 37734011
If you are over allocated more vcpu just makes it worse
Only add vcpu not counting hyperthreading.
It really depends on your boxes and vms how to best set this up. For example, I have a osx vm for Xcode compiling runnin on an Xserve. I gave it 4 vcpu because it can compile multithreaded.
I also have a mailserver that would run at 90% inside the vm with one vcpu which was basically maxing one core of the real CPU. So I gave it two vcpu and now it's running at 30%. Whatever, it's happy. The general rule is as few as possible. Most of my vms are 1vcpu.
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