linux-kernel - Very bad performance on Qemu/KVM

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Date:	Tue, 16 Oct 2012 09:51:22 +0200
From:	Nico Schottelius <nico-kernel20121016@...ottelius.org>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: Very bad performance on Qemu/KVM

Good morning LKML,

I see extremely slow virtual machines
running on qemu-kvm-0.12.1.2-2.295.el6_3.2.x86_64
on Centos 6.3 with Linux 2.6.32-279.5.2.el6.x86_64 on a
64 core machine using AMD Opteron(TM) Processor 6276 (dell r815).

The guests are Centos 6.3, Linux 2.6.32-220.13.1.el6.x86_64.
The disk images reside on local storage, ext4 filesystem.

I checked that kvm is loaded and enabled. I changed
-drive to use if=virtio, -net to use model=virtio.

>From the processes running it seems that cpu performance is the
bottleneck. I added -cpu host to skip the emulation, but this did
not fix the issue.

Comparing the performance to qemu running on a xenserver, performance is
at most half of it.

Does anyone of you have a good pointer how to debug this further or are
there any known issues with the given setup? 

I'm happy for every pointer I get, as there seems to be no easy to spot
issue.

Cheers,

Nico

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