On 9/11/2009 at 3:53 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> 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: > > # Samples: 98703304 > # > # Overhead Command Shared Object Symbol > # ........ ............... ................................. ...... > # > 97.15% sh [kernel] [k] > vmx_vcpu_run > 0.27% sh [kernel] [k] > kvm_arch_vcpu_ioctl_ > 0.12% sh [kernel] [k] > default_send_IPI_mas > 0.09% sh [kernel] [k] > _spin_lock_irq > > Which is pretty good. Without EPT/NPT the mmu_lock seems to be the major > bottleneck to parallelism. > >> 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. I suspected it might be a whole lot of spinning happening. That does seems most likely. I was just surprised how bad the behavior was. Bruce -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
