Using all cores to 100% on host

[geohei]

Hi.

I'm presently working on a test project with VirtualBox.

Host: Windows 10 on i9900 CPU
Guest : Ubuntu 18.04 server
VB: 6.0.14

I compiled some code using the POSIX multithread libraries (pthread.h).
When using 16 threads, I get around 62% CPU load on the host side.

For testing, I recompiled using 8 and 32 threads.
More or less the same result.
The host CPU load varies between 55% and 63%.

Is that normal?

[Martin]

You have a CPU with 8 cores.
The hosts needs a free core for itself to run a VM efficiently.
So in most cases you would see best guest performance with less than 8 virtual CPUs in the guest on this system.
Threds don't count for virualization in almost all cases.

[scottgus1]

also, the more cores you put in a guest the slower it will run, due to extra scheduling commands needed on the host to put the guest commands through the CPU cores. A modern OS is happy with two cores, and more will make it sluggish unless there are other multi-threaded programs running in it that need more cores.

[geohei]

Thanks a lot for the answers.

You have a CPU with 8 cores.
The hosts needs a free core for itself to run a VM efficiently.
So in most cases you would see best guest performance with less than 8 virtual CPUs in the guest on this system.
Threds don't count for virualization in almost all cases.

For testing purpose, I reduced from 8 to 7 cores.
Performance dropped from 63% to 51%.

also, the more cores you put in a guest the slower it will run, due to extra scheduling commands needed on the host to put the guest commands through the CPU cores. A modern OS is happy with two cores, and more will make it sluggish unless there are other multi-threaded programs running in it that need more cores.

That's what I noticed as well (2 cores are best), but only for single-thread binaries. Mine is however multi-thread and uses all cores.