Re: [Qemu-devel] kvm suspend performance

[Thomas Knauth]

Hi Stefan,

thanks for taking the time to reply.

On Wed, Mar 20, 2013 at 9:11 AM, Stefan Hajnoczi <address@hidden> wrote:

Which QEMU or libvirt command are you using to suspend the guest to

disk?

virsh save <name> <file>

 

Why do you say it is CPU-bound?  Did you use a tool like vmstat or
simply because it does 30 MB/s instead of the expected 100 MB/s?

I monitor the system with top/vmstat. The kvm process shows a utilization of 16% while the libvirtd process has a utilization of 4-5%. As this is a 4-core machine, a utilization of 25% would indicate that one core is fully utilized. vmstat also gives the number of context switches. I see a background context switch rate of about 60. This increases by 10x, to around 600, during the save. But even 600 cxt switches per second doesn't feel like a bottleneck.

So while I agree, that I might have jumped to quick to a conclusion about the CPU-boundedness, there has to be some bottleneck. I would like to know where and why. In my naive view we are just reading memory and writing it to disk. This should progress faster than the 30 MB/s I am seeing right now.

Which versions of libvirt and QEMU are you using?

I've tried this on a couple of machines with differing versions of Ubuntu (12.04, 10.10, and 10.04). They are all showing the same performance. The machine where I did most of my measurements was the 12.04 machine. The versions are libvirt 0.9.8-2ubuntu17 and 

qemu-kvm 1.0+noroms-0ubuntu14.7.

Kind regards,

Thomas.