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I am using qemu with kvm as my hypervisor. I am running a gentoo guest on an ubuntu host. The guest is being given a single processor.

When I run a compute intensive job (say empty-loop) I get almost 100% CPU usage on guest as well as host. However, when I play a high resolution video ( 1080p) I see that the guest is showing upto 100% CPU usage whereas the host is showing only about 30-40% average CPU usage. I wonder what could be causing this.

If the process was Disk bound, then I believe a second run of the video would have improved the host-to-guest CPU usage ratio, as the video would have been fetched to the main memory ( the video is approx 1 minute, 40 MB and the guest ram is 4 gigs), but that does not happen. Then what could cause this difference in CPU usage ?

I dont fully understand how vlc handles H264 videos. It reports that it is dropping frames possibly due to slow computer. My only guess is that perhaps a graphics card is involved that the qemu waits for and the guest is not aware of this waiting.

My host machine has 12 cores with 24 gigs of RAM.


The problem had nothing to do with running a virtual machine. It was because I was accessing my test machine through SSH. X11 forwarding was causing a major performance slowdown to my qemu process. Perhaps it is similar to how a normal program slows down if we have too many printf statements. The network was becoming a bottleneck in my case.

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Is the host showing 30-40% usage of one core, or overall? – Ben Voigt May 7 '12 at 16:02
@Ben the 30-40% usage is on one core .. – AnkurVj May 7 '12 at 17:26
Are all supported virtualisation technologies by the cpu like VT-D,Vt-X,IOMMU turned on? – D.Iankov May 8 '12 at 9:15
@D.ian I found out the reason for what I was observing, I have added an edit to that effect. – AnkurVj May 8 '12 at 21:28

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