performance tuning, problem with paravirtualized clock

[Nikola Ciprich <extmaillist@linuxbox.cz>]

Hi,
I spent some time trying to tune performance of KVM guest using kernel 
compilation as a kind of benchmark (I'm using virtual machines for 
compiling a lot, so it's good benchmark for me in general)

Host machine: 2x quad core XEON E5420  @ 2.50GHz, 4GB RAM, 2.6.24 + kvm-64
guest configuration: all 8 cores available, 2GB  RAM, 2.6.24 or latest GIT 
+ kvm-64

some results:
- compilation in KVM guest is roughly 2x slower than on bare metal.
- trying various block device backends (ide, scsi, virtio_blk) didn't 
really matter much for my case
- enabling CONFIG_KVM_GUEST under latest GIT with kvm-64 patch applied 
decreased compile time by about 10%, which is nice!
- enabling CONFIG_KVM_CLOCK made guest unstable, often unable to finish 
booting at all, disabling acpi made it a bit better, but still quite 
unstable (cpu0 lock-ups, etc)

Is there currently anything more I could do to improve performance? I'm 
wandering what is slowing compilation, if I compare some CPU intensive 
application (ie bzip2), it seems to run in nearly native speed, but kernel 
compilation is much slower even if run from ramdisk, maybe it could be 
improved further by tunning scheduler etc?

anyways keep up the good work!
cheers!
nik


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