KVM causes high CPU loaded when cache=’none’

cpu-usagekvm-virtualizationvirtualization

I've followed instructions on http://www.linux-kvm.org/page/Tuning_KVM.
Host is Debian Squeeze Kernel 3.2, QEMU 1.0, libvirt 0.9.12 (all from squeeze-backport). All 4 guests are Debian Squeeze Kernel 3.2, also.

So my settings in Guest XML are

<cpu model='host-passthrough'/>
<disk [...]/>
   <driver name='qemu' type='raw' cache='none'/>
   <target [...] bus='virtio'/>
</disk>
<interface [...]>
   <model type='virtio'/>
</interface>

IO scheduler on guests is set to noop. On host I tried noop/deadline/cfq with no significant performance differences, for me. All guests storage is provided by LVM. When using cache='none' and all guests have no noteworthy load, the 15 minutes average CPU load on host goes up to 3-4. But when using cache='writeback' the hosts CPU loaded rises down to less than 1. Can anyone explain, why the suggested settings for LVM causes the higher load on host?

BTW: When running disk benchmarks, the option none results in higher IO performance than writeback.

Best Answer

OK, with additional option io='native' in disk section and IO scheduler cfq on host system, I'll get the best results for my system. IO rate is nearly the same for all values of option io in guest XML and for IO scheduler on host and guest, only cache='unsafe' gives significantly more performance. But only with io='native', noop scheduler in guest and cfq scheduler on host I'll get lowest CPU load.

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