Use dummynet in a VM? High latency? Set kern.hz to 1000

As a hobby, I play with software routers in virtual machines (always FreeBSD). One recent project of mine was to emulate DSL bandwidth and latency in a VM, from varying bandwidths of 1.5 Mbps (ADSL) to 50 Mbps (VDSL2).

By default, when using FreeBSD on a hypervisor, the kern.hz tunable is set to 100. This is to prevent additional CPU use from idling.

But dummynet recommends kern.hz=1000 which is also the default on physical hardware. This is to prevent additional latency as dummynet uses clock ticks, and this also works with VMs.

Using dummynet and the default kern.hz=100 on both VirtualBox and bhyve hosts had about 10-20ms delay.

If I set kern.hz=1000 in /boot/loader.conf, the latency from dummynet drops to <1ms on average, and at worst 1-2ms.

However, this has the disadvantage of higher idle CPU use (as mentioned earlier) in the VM running dummynet. I only recommend setting kern.hz=1000 in a VM only if you are using something like dummynet which relies on clock ticks and some idle CPU use is acceptable.