Re: nested KVM slower than QEMU with gnumach guest kernel

[Jan Kiszka <jan.kiszka@web.de>]

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On 2014-11-11 19:55, Samuel Thibault wrote:
> Hello,
> 
> jenkins.debian.net is running inside a KVM VM, and it runs nested
> KVM guests for its installation attempts.  This goes fine with Linux
> kernels, but it is extremely slow with gnumach kernels.  I have
> reproduced the issue with my laptop with a linux 3.17 host kernel, a
> 3.16 L1-guest kernel, and an i7-2720QM CPU, with similar results; it's
> actually even slower than letting qemu emulate the CPU... For these
> tests I'm using the following image:
> 
> http://people.debian.org/~sthibault/tmp/netinst.iso
> 
> The reference test here boils down to running qemu -cdrom netinst.iso -m
> 512, choosing the "Automated install" choice, and waiting for "Loading
> additional components" step to complete. (yes, the boot menu gets
> mangled ATM, there's apparently currently a bug between qemu and grub)
> 
> My host is A, my level1-KVM-guest is B.
> 
> KVM:
> A$ qemu -enable-kvm -cdrom netinst.iso -m 512M
> takes ~1 minute.
> 
> QEMU:
> A$ qemu             -cdrom netinst.iso -m 512M
> takes ~7 minutes.
> 
> KVM-in-KVM:
> B$ qemu -enable-kvm -cdrom netinst.iso -m 512M
> takes ~10 minutes, when it doesn't gets completely stuck, which is quite
> often, actually...
> 
> QEMU-in-KVM:
> B$ qemu             -cdrom netinst.iso -m 512M
> takes ~7 minutes.
> 
> I don't see such horrible slowdown with a linux image.  Is there
> something particular that could explain such a difference?  What tools
> or counters could I use to investigate which area of KVM is getting
> slow?

You can try to catch a trace (ftrace) on the physical host.

I suspect the setup forces a lot of instruction emulation, either on L0
or L1. And that is slower than QEMU is KVM does not optimize like QEMU does.

Jan


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