hurdle
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Limits on CPU

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I have found a ton of stuff ast to the Pro's and Con's or setting Limits on Memory in a resource pool.  For example that fact that a VM will never go past its configured memory  overhead, ect.

But I have not found much of anything on effect of putting Limits on CPU.

Thoughts?

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rickardnobel
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hurdle wrote:

But I have not found much of anything on effect of putting Limits on CPU.

A CPU limit of a certain number of Mhz means that the VM could never use more than those number of clock cycles per second, even if there are lots of them available. This means that it is very likely that this VM will suffer from bad performance, perhaps also very un-necessary.

Using CPU Shares could be a better way to give the VM a lower priority against other VMs, but still let it use CPU if it is available.

My VMware blog: www.rickardnobel.se

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sparrowangelste
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one case where a limit on a cpu might be useful is if the host is overcommited and there are vms that have less important , so placing a limit on a vm might limit contentionready values for other more critical vms

also limits might be useful for a dev situation where you want to limit the speed to match a slower system.

also read this: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=103311...

--------------------- Sparrowangelstechnology : Vmware lover http://sparrowangelstechnology.blogspot.com
rickardnobel
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hurdle wrote:

But I have not found much of anything on effect of putting Limits on CPU.

A CPU limit of a certain number of Mhz means that the VM could never use more than those number of clock cycles per second, even if there are lots of them available. This means that it is very likely that this VM will suffer from bad performance, perhaps also very un-necessary.

Using CPU Shares could be a better way to give the VM a lower priority against other VMs, but still let it use CPU if it is available.

My VMware blog: www.rickardnobel.se

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jklick
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I would concur that shares are generally a better way to go than limits. I've seen too many circumstances where limits unneccesarily cause CPU ready issues, unbeknownst to the admin.

As a side note, if you're ever in a situation where you're encountering CPU ready issues and you want to know if it's general contention (caused by high vCPU:pCPU ratios) or if it's caused by a CPU limit (either at the VM or resource pool level), there's a metric in ESXTOP called %MLMTD that measures this.

@JonathanKlick | www.vkernel.com