I am doing some tests on the performance of QEMU (qemu-system-i386) in full emulation mode (with TCG), and avoiding KVM. I have installed the version available in the debian repository (QEMU emulator version 1.1.2 (Debian 1.1.2+dfsg-6a+deb7u6), and downloaded and compiled the sources for Qemu 2.3.0, Qemu 1.0.0, Qemu 1.1.2, and also the source for Debian 1.1.2+dfsg-6a+deb7u6.
I am configuring the project in the following way, in all cases: ./configure --enable-sdl --target-list=i386-softmmu --disable-kvm --enable-tcg-interpreter
The version installed from the repository is quite faster than any of the versions manually compiled. I am always using the same image for the hard drive). I have ensured that kvm is never enabled or loaded: * Querying qemu (info kvm) * Looking for loaded drivers (lsmod | grep kvm), and (ls /dev/kvm).
Kvm is not loaded in any of the cases.
I have also tried to change the -O2 by -O3 in the configuration file, and stripped the symbols in the binary (which should not be a difference).
I must be missing something (default configuration for the debian package, compilation options... but I cannot figure out what).
Which could be the reason for this performance difference? (Any idea, experience?)
Thank you very much!
The problem here is your '--enable-tcg-interpreter' argument to configure. This disables the usual JIT-based TCG backend for the host CPU in favour of a slow interpreted backend. You never want the interpreter unless QEMU would otherwise not support your host CPU at all; as you have found, it is markedly slower than the default JIT.