[Xen-users] VCPU: Xen vs VMWare ESX

[James Pifer <jep@xxxxxxxxxxxxxxxx>]

We currently use Xen on SLES11SP2. Some want us to switch to VMWare ESX. 
I have a VCPU question in regards to how each platform handles CPU 
scheduling. I did some searching, but rather trying to read a bunch of 
web pages I'm hoping someone here can give me an answer.
I have a Xen server (HP DL360 G8p), Dual Xeon with 24 VCPUs. I'm 
currently running 26 VMs with the following CPU assignments:
14 VMs have 4 vcpus
12 VMs have 2 vcpus
1 VM has 1 vcpu

Obviously I'm over subscribing the vcpus and count on the scheduler to 
handle it.
A VMWare guy is telling me you have to be very careful with VMWare to 
properly size maximum VM to host ratio or you'll end up with VMs waiting 
for CPU. If you assign 4 CPUS to a VM, all four CPUs must be available 
or the VM will wait. Maybe that's the same in Xen and there's enough 
idle time to handle it (on my server)?
Maybe my Xen VMs are waiting on CPUs, I'm not sure, but I've never had a 
complaint about the performance on this host for any of the VMs.
Can anyone with both Xen and VMWare experience comment?

Much appreciated.

Thanks
James

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