KVM vs. Kqemu

Yesterday I tried kvm on my macbook, because I recognized that the cpu does support hardware based virtualization. I wanted to run a virtual machine with the actual kde4 release candidate. First I tried the Suse based KDE4Live, but kvm crashed immediately with “exception 13”. I dunno why that happened, but I didn’t get it working. Then I tried the debian based KDE4-LiveCD which is unfortunately only based on KDE4 beta4. But that cd worked. 🙂
I was just wondering if kvm is quite faster than qemu with kqemu, because it didn’t felt faster. I know that kvm does have a few advantages compared with kqemu, like smp and hardware visualization support, but I asked myself how much faster (or actually slower) would it be compared to kqemu.
So I set up a minimal Debian/testing installation in a qemu image. After that I tried in snapshot mode how long it would last to compile a new standard debian kernel once with kvm and once with kqemu.
But enough words are spoken. Let’s see the facts:
Test system:
1,83 Ghz Core Duo MacBook as host with kernel 2.6.22-1-686.
The VM got:
– 256 MB RAM
– 2.5GB HDD space

startup time:
kvm: 30 sec
kqemu: 42 sec

kernel compile time:
kvm: 1h 27min 57sec
kqemu: 2h 13min 17sec

All right. kvm is a lot faster than kqemu, so the reason that it feels not faster on my macbook seems to be the small amount of ram. So I need more ram for my macbook. 😉


  1. That’s a tad ‘funny’. I’ve done extensive performance testing of bare qemu vs qemu with kvm vs qemu with kqemu, and I’ve found on all account kqemu is actually superior. Both in performance and stability (specially stability!) The problem being, while bare number crunching is slightly faster on kvm (now anyway, with v69. was much slower pre-v54) was that kvm had severe IO latency issues with both disk io and network io, to the point of it almost being useless for anything apart from home hobby use. Combined with the /severe/ instability on x86_64 and inability to address more than 4gb, even when running 64bit host and 64bit guests, I’ve pretty much given up on kvm as being a passing fad that just won’t survive the real world.

  2. Well it seems you were terribly wrong with that comment 😀

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