[Qemu-discuss] Puzzling performance comparison with KVM and Hyper-V

[Tim Bell]

 

We are running a compute intensive application on a variety of virtual machines at CERN (a subset of Spec 2006). We have found two puzzling results during this benchmarking and can’t find the root cause after significant effort.

 

1.      Large virtual machines on KVM (32 cores) show a much worse performance than smaller ones

2.      Hyper-V overhead is significantly less compared to KVM

 

We have tuned the KSM configuration with EPT off and CPU pinning but the overheads remain significant.

 

4 VMs 8 cores:  2.5% overhead compared to bare metal

2 VMs 16 cores: 8.4% overhead compared to bare metal

1 VM 32 cores: 12.9% overhead compared to bare metal

 

Running the same test using Hyper-V produced

 

4 VMs 8 cores: 0.8% overhead compared to bare metal

1 VM 32 cores: 3.3% overhead compared to bare metal

 

Can anyone suggest how to tune KVM to get equivalent performance to Hyper-V ?

 

Configuration

 

Hardware is Intel(R) Xeon(R) CPU E5-2650 v2 @ 2.60GHz, SMT enabled, 2GB/core

CentOS 7 KVM hypervisor with CentOS 6 guest

Windows 2012 Hyper-V hypervisor with CentOS 6 guest

Benchmark is HEPSpec, the c++ subset of Spec 2006

The benchmarks are run in parallel according the number of cores. Thus, the 1x32 test runs 32 copies of the benchmark in a single VM on the hypervisor. The 4x8 test runs 4 VMs on the same hypervisor, with each VM running 8 copies of the benchmark simultaneously.