[Qemu-discuss] Why do additional cores reduce performance?

[Oleg Ovechko]

Hi,

I am novice to kvm/qemu, perhaps you can explain something to me.
I pass-through sata controller with fakeraid. It works, but I am getting
strange results during performance testing:

A. Host Windows, 6 cores (no HT, turbo boost off): 6:23 (+- 10 secs)
B. Host Windows, 1 CPU core (other are turned off in BIOS): 7:13 (+-10 secs)
C. Host 1 core, Guest Windows 1 core: 7:15 - same as B, no degradation
D. Host 6 cores, Guest Windows 1 core: 7:57
E. Host 6 cores, Guest Windows 4 cores: 8:17

Perhaps I am doing something very wrong, D->E looks very surprising to me,
and C->D is just unbelievable. I am adding cores and it works worth. How
can it be?

1. What is correct way of passing CPU cores to VM?
2. Can I assign CPU core solely to VM to avoid cache resets and context
switching to Host?
3. Also I am unsure about HT. When I specify "cores=2", is there any
guaranty that whole core with both HT parts is passed to VM? Or it can be
mix of two real cores with separate caches?
4. Any other comment on qemu command line?

I am using Ubuntu 14.10, Qemu 2.1.95, Windows 2012R2
uname -a Linux meta 3.16.0-28-generic #37-Ubuntu SMP Mon Dec 8 17:15:28 UTC
2014 x86_64 x86_64 x86_64 GNU/Linux
Hardware is: i7 5930K, AsRock X99 Professional

Here is my qemu command line:

qemu-system-x86_64 -machine type=pc -nodefaults -m 16G -enable-kvm \
-cpu host -smp 4,sockets=1,cores=4,threads=1 \
-rtc base=localtime,clock=host \
-bios /home/oleg/seabios175.bin \
-hda /home/oleg/windows.img \
-device vfio-pci,host=00:1f.0,addr=1f.
0,multifunction=on \
-device
vfio-pci,host=00:1f.2,addr=1f.2,romfile=/home/oleg/8086-2822_v13.2.0.2134_patched.bin
\
-device vfio-pci,host=00:1f.3,addr=1f.3 \
-vga std