We are using Ubuntu 12.04 with the following parameters:
- Dell R910
- Kernel 3.2.0-25-generic #40-Ubuntu SMP x86_64 x86_64 x86_64 GNU/Linux
- kvm 1:84+dfsg-0ubuntu16+1.0+noroms+0ubuntu13
- qemu-kvm 1.0+noroms-0ubuntu13
- qemu-common 1.0+noroms-0ubuntu13
- qemu-kvm 1.0+noroms-0ubuntu13
- 4 x Intel(R) Xeon(R) CPU E7- 4870 @ 2.40GHz (each with 10 physical cores, HT and Intel VT enabled)
- The Windows guests currently have no VirtIO, but that will change soon
We are running several Windows guests on this machine, one of them is Windows 2003 32 Bit, another one Windows 2008 (64 Bit). We are currently struggling with performance issues and played around with the CPU models.
We usually use "qemu-system-x86_64 for our Windows 32 Bit guest, e.g.:
/usr/bin/qemu-system-x86_64 -S -M pc-1.0 -cpu qemu32 -enable-kvm -m 4096 -smp 4,sockets=4,cores=1,threads=1 [...]
The performance of this guest turned out to be a bit low. We didn't run any benchmark yet, but let's say that copying large amount of data (files) inside the VM from one directory to the other goes much faster when we switch the CPU model from "-cpu qemu32" to "-cpu Nehalem". Files which took around 2:40h to copy now copy within 40 minutes. Of course this is not a high quality test and there is much room for having a more professional attempt. But this is a clear indicator that choosing the correct CPU model could affect the guest's performance heavily.
Now I got curious and ran:
qemu-x86_64 -cpu ? x86 [n270] x86 [athlon] x86 [pentium3] x86 [pentium2] x86 [pentium] x86  x86 [coreduo] x86 [kvm32] x86 [qemu32] x86 [kvm64] x86 [core2duo] x86 [phenom] x86 [qemu64]
kvm -cpu ?model x86 Opteron_G3 AMD Opteron 23xx (Gen 3 Class Opteron) x86 Opteron_G2 AMD Opteron 22xx (Gen 2 Class Opteron) x86 Opteron_G1 AMD Opteron 240 (Gen 1 Class Opteron) x86 Nehalem Intel Core i7 9xx (Nehalem Class Core i7) x86 Penryn Intel Core 2 Duo P9xxx (Penryn Class Core 2) x86 Conroe Intel Celeron_4x0 (Conroe/Merom Class Core 2) x86 [n270] Intel(R) Atom(TM) CPU N270 @ 1.60GHz x86 [athlon] QEMU Virtual CPU version 1.0 x86 [pentium3] x86 [pentium2] x86 [pentium] x86  x86 [coreduo] Genuine Intel(R) CPU T2600 @ 2.16GHz x86 [kvm32] Common 32-bit KVM processor x86 [qemu32] QEMU Virtual CPU version 1.0 x86 [kvm64] Common KVM processor x86 [core2duo] Intel(R) Core(TM)2 Duo CPU T7700 @ 2.40GHz x86 [phenom] AMD Phenom(tm) 9550 Quad-Core Processor x86 [qemu64] QEMU Virtual CPU version 1.0
With all these different versions, it's a bit hard to guess at. "Nehalem" seems to be the most performant one on that list. Now I wonder, how to tell which CPU model is the best for my guest? Browsing the Internet, I found the following ressources:
When I read those sites correctly, they are claiming that "-cpu host" might bring the best performance. I don't have any worries about migration yet, since both KVM hosts are equipped equally (exactely the same hardware).
So, what do experienced KVM admins recommend? Is there a golden rule or even a matrix, like "this model is the best for that guest OS"?
My apologies if I could find out this information by my own - I ran various Google searches and browsed many websites. I was not able to find something which answers my question.