![]() Often, VMs will be given resources just because, or "Because the vendor says so", then wonders why they have this situation occur.Īlso, hyperthreading is not your friend because despite popular belief, it's not a full added thread in reality it's only a 50% increase in performance. This is why the term "Right Sizing" is so heavily stressed and used, you have to properly size the VM's resources to the actually workload and observe. Now one may think to just throw more vCPU at the problem, but that only makes the situation worse for not only the VM in question, but all VMs on the host. Often, if the storage doesn't have high latency, this will be something in the network stack, like a long running SQL query or a large single-threaded data transfer. If this is high, then the VM is waiting for the vCPU to process IO from either the storage or network stack, which causes the watchdog threads to take much longer to complete, therefore the VM thinks it's CPUs are heavily utilized, when in fact that is not the case, you have an IO bottleneck somewhere. One thing to keep in mind is that for the most part, the vCPU of the VM will be used to process data that normally would be handled by a storage controller and NIC, which is the IOWAIT measurement. Now, there's a whole slew of other factors in this, %RDY, IOWAIT, CO-STOP, etc. The VM guest has no knowledge of this so therefore is blind and thinks that things are more utilized than they are. In this case, the host is actually correct because it knows about the scheduling and can take that into account, as well as all the other VMs running on it. ![]() Taking into account the vCPU's are scheduled across a finite number of pCPU's, and that other VM workloads can impact this, you often will see a VM guest reporting higher CPU usage than what the host itself reports for the same VM. So, you have a VM that is reporting high CPU usage because it's timer threads are taking a while to return. This is what happens regardless of the hardware, which is why it works because it is hardware agnostic. The logic is that the low priority thread will not complete until all other threads are completed, so in theory, this is a viable ball park measurement. That calculation is the measurement for usage of the CPU. It does this by starting a low priority threat on the CPU(s) and waits to see how long it takes for that thread to complete. The reason is there's different methods measured in order to come up with the measurements.įor instance, most OS's (Linux and Windows) use a method called a Watchdog Timer in order to determine the CPU usage. So you're saying the only indication of throttling you have is that the guest numbers don't match the host numbers? Well, they won't, rarely ever.
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