Marcelo Tosatti wrote:
On Fri, Sep 11, 2009 at 09:36:10AM -0600, Bruce Rogers wrote:
I am wondering if anyone has investigated how well kvm scales when supporting many guests, or many vcpus or both.
I'll do some investigations into the per vm memory overhead and
play with bumping the max vcpu limit way beyond 16, but hopefully
someone can comment on issues such as locking problems that are known
to exist and needing to be addressed to increased parallellism,
general overhead percentages which can help provide consolidation
expectations, etc.
I suppose it depends on the guest and workload. With an EPT host and
16-way Linux guest doing kernel compilations, on recent kernel, i see:
> ...
Also, when I did a simple experiment with vcpu overcommitment, I was
surprised how quickly performance suffered (just bringing a Linux vm
up), since I would have assumed the additional vcpus would have been
halted the vast majority of the time. On a 2 proc box, overcommitment
to 8 vcpus in a guest (I know this isn't a good usage scenario, but
does provide some insights) caused the boot time to increase to almost
exponential levels. At 16 vcpus, it took hours to just reach the gui
login prompt.
One probable reason for that are vcpus which hold spinlocks in the guest
are scheduled out in favour of vcpus which spin on that same lock.
We have encountered this issue some time ago in Xen. Ticket spinlocks
make this even worse. More detailed info can be found here:
http://www.amd64.org/research/virtualization.html#Lock_holder_preemption
Have you tried using paravirtualized spinlock in the guest kernel?
http://lkml.indiana.edu/hypermail/linux/kernel/0807.0/2808.html
Regards,
Andre.
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