Hello,
According to https://www.linux-kvm.org/page/Lists,_IRC it's OK to use
this list for KVM user questions, so here it goes:
How important is it to consider "co-stop" with KVM?
I haven't been able to find anything about this for KVM, but there's
some material about it for VMWare (albeit typically rather old
material). Based on VMWare material, by "co-stop" I mean the following
situation:
A VM called "x" has been assigned quite a few vCPUs, e.g. 10, on a
hypervisor with e.g. 20 physical cores. The hypervisor is hosting many
other VMs, and the many VMs take turn running on the hardware.
Now, if it's often the case that, e.g., only 7 physical threads are
available when it's x's turn to run, the hypervisor needs to postpone
running x till all 10 cores can be allocated at the same time. During
this waiting time, x is stopped (co-stop).
Is my understanding correct? Or will KVM allow x to run on (e.g.) 7
cores, even though the VM thinks it has 10 vCPUs available?
It's my understanding that with KVM, the co-stopped situation is
reflected in the VM's "steal time" metric in a tool like "top".
Does hyperthreading in either the hypervisor and/or the guest impact the
risk of co-stop?
--
Regards,
Troels Arvin
