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We recently had a vmware consultant come in to do a health check. One of the things he mentioned briefly that we should change is that we should remove the cpu limitations that we have set for the VMs. His reasoning is that it causes unneccesary over head on the ESX servers.
Wondering what everyones thoughts were on this, I thought the cpu limitation was a good thing because it stopped a vm from over using cpu....Any advice is much appreciated.
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I do not think it will cause overhead for the CPU, limits are useful in a few scenarios but should be carefully thoughtover and not be implemented by default.
In your case I think its a better idea to implement reservations and shares.
// Linjo
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Its always good practice to set CPU limits across VMs. When CPU resources are overcommitted, the host time-slices the physical processors across all virtual machines, so each virtual machine runs as if it has its specified number of virtual processors. When host runs multiple virtual machines, it allocates to each virtual machine a share of the physical resources.
There are two things here:-
1) When all VMs are running and contending for the CPU cycles then in that case cpu cycles will be divided as per the share across all the VMs.
2) When one VM is running and rest are idle(not active) then in that case active VM can grab whatever CPU cycles it requests.
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I don't agree with shishir08 here, limits should be carefully thoughtover and only be implemented for specific vm:s.
Why limit a machine if there are resources available for it to consume?
I think however that shares and reservations always should be implemented.
// Linjo
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I agree with Linjo. I only use limits in very specific instances where a VM continually has a problem where it takes the CPU up to 100% for a period of time and also if that VM is not considered critical. Shares/Reservations are a much better way to control the time slices that a VM gets so that it doesn't cause havoc on your hosts. To me limiting is a last resort.
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thanks....makes sense