Program is slower on VM than on physical node

asked 2018-12-13 18:40:18 -0500

calm07 gravatar image

I am observing noticeably faster speeds on a physical node when compared to a VM. More details are below.

I have a complicated C++ program that uses OpenMP. It runs at an average of 4-5 mins on a physical node. When I use the same physical node as a compute node and launch a VM on it, it runs at an average of 10-11 minutes. I understand that virtualization will add some overhead but currently the physical node is 2x faster than the VM. Is that the best I can get with virtualization? If I run "top" on the VM, I notice that it uses less CPU% than compared to the physical node.

If I run the program on an Amazon AWS instance, it runs at an average of 4-5 minutes. So it seems to me that some optimization can be done to make my VM faster. I have tried CPU pinning with no noticeable improvement.

There is only one related question in the OpenStack forums - ( But I have already implemented the solution provided there.

I would be very grateful if someone could point me in the right direction. Right now, I am not sure how to debug the issue.

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You don't know how fast the program would run on the physical server used by AWS.

What hypervisor is configured? On the compute node, check nova.conf. Most probably it's libvirt; in which case what is the value of the virt_type config parameter? If qemu, your cloud emulates CPUs, thus slow.

Bernd Bausch gravatar imageBernd Bausch ( 2018-12-13 20:25:05 -0500 )edit