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how can i easily tell if i have too many cpus and memory in my virtual server

Posted on 2014-08-28
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Last Modified: 2014-09-26
when developers ask for new servers to be provisioned they say they must have 6 cpus or 12 cpus
we try to tell them that it is best practice to strat with 1 and work from there
however some software installation engineers refuse to carry out the install unless they have the cpu count they have asked for.
How can i easily prove to them that they have too many cpus and too much memory
or that their vm is actually performing worse due to the high cpu count they have
cpu ready etc is hard to explaint and show them
is there anything that i can use that will easily prove less is better
we have machines running with too many cpus and too much ram
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Question by:dougdog
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LVL 125
ID: 40289942
If the server was physical, would they need 6 CPU or 12 CPU machines?

because I've not seen 6 or 12 physical CPU machines.

It's a well known fact in "hypervisor circles!"

print this out.....

vSMP (virtual SMP) can affect virtual machine performance, when adding too many vCPUs to virtual machines that cannot use the vCPUs effectly, e.g. Servers than can use vSMP correctly :- SQL Server, Exchange Server.

This is true, many VMware Administrators, think adding lots of processors, will increase performance - wrong! (and because they can, they just go silly!). Sometimes there is confusion between cores and processors. But what we are adding is additional processors in the virtual machine.

So 4 vCPU, to the VM is a 4 Way SMP (Quad Processor Server), if you have Enterprise Plus license you can add 8, (and only if you have the correct OS License will the OS recognise them all).

If applications, can take advantage e.g. Exchange, SQL, adding additional processors, can/may increase performance.

So usual rule of thumb is try 1 vCPU, then try 2 vCPU, knock back to 1 vCPU if performance is affected. and only use vSMP if the VM can take advantage.

Example, VM with 4 vCPUs allocated!

My simple laymans explaination of the "scheduler!"

As you have assigned 4 vCPUs, to this VM, the VMware scheulder, has to wait until 4 cores are free and available, to do this, it has to pause the first cores, until the 4th is available, during this timeframe, the paused cores are not available for processes, this is my simplistic view, but bottom line is adding more vCPUs to a VM, may not give you the performance benefits you think, unless the VM, it's applications are optimised for additional vCPUs.

See here
http://www.vmware.com/resources/techresources/10131

see here
http://www.gabesvirtualworld.com/how-too-many-vcpus-can-negatively-affect-your-performance/

http://www.zdnet.com/virtual-cpus-the-overprovisioning-penalty-of-vcpu-to-pcpu-ratios-4010025185/

also there is a document here about the CPU scheduler

www.vmware.com/files/pdf/perf-vsphere-cpu_scheduler.pdf

https://blogs.vmware.com/vsphere/2013/10/does-corespersocket-affect-performance.html
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Author Comment

by:dougdog
ID: 40289999
we have a oracle server that has 24 cores
when configuring is there any majot difference between
adding them as 1 virtual socket with 6 cores or 6 virtual sockets with 1 core
0
 
LVL 125
ID: 40290011
No difference....

Cores is useful if you have license restrictions....

otherwise I would stick to using sockets....

last link of my first post...

https://blogs.vmware.com/vsphere/2013/10/does-corespersocket-affect-performance.html
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Author Comment

by:dougdog
ID: 40292114
what is classed as high cpu ready and co-stop for a vm
does this still apply as much to 5.5
0
 
LVL 125
ID: 40292136
Applies to all Hypervisors.

from the article:-

READY: The time a virtual machine must wait in a ready-to-run state before it can be scheduled on a CPU
CO-STOP: Amount of time a SMP virtual machine was ready to run, but incurred delay due to co-vCPU scheduling contention.

http://www.gabesvirtualworld.com/how-too-many-vcpus-can-negatively-affect-your-performance/
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Author Comment

by:dougdog
ID: 40292281
yes but i mean what would be classed as a high cpu ready and co stop time in ms
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LVL 125

Accepted Solution

by:
Andrew Hancock (VMware vExpert / EE MVE^2) earned 2000 total points
ID: 40292323
thresholds... (but also read the yellow brick article!)

%RDY > 10

%CSTP > 3

this article better explains it...

Troubleshooting a virtual machine that has stopped responding: VMM and Guest CPU usage comparison (1017926)

also see here

http://www.yellow-bricks.com/esxtop/
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