Hello, How important is it to consider "co-stop" with non-VMWare virtualization such as 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
Hi Arvin, If you don’t do cpu-pinning and you overcommit cpu resources. KVM will schedule cpu load to best effort. I always overcommit cpu resources because vcpu’s are rarely on 100% load and KVM will divide actual vcpu load over the physical cpu’s. On rare occasions there is not enough physical capacity available and the users notice ‘lag’ on the systems. I think VMware does the same and co-stop is just a performance metric to measure this effect, but I’m not well known with VMware. I found this post though: https://blog.leaseweb.com/2023/01/10/performance-metrics-in-vmware/ Greetings, Dominique. Van: Troels Arvin via Users <users@lists.libvirt.org> Verzonden: maandag 8 september 2025 15:05 Aan: users@lists.libvirt.org Onderwerp: Co-stop with non-VMware virtualization? Hello, How important is it to consider "co-stop" with non-VMWare virtualization such as 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
participants (2)
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Dominique Ramaekers -
Troels Arvin