Subj : Arch btw
To   : Accession
From : Vorlon
Date : Wed Dec 03 2025 11:19:42


Hello Accession!

02 Dec 25 17:02, you wrote to me:

 >> Where are you getting that information from? My server's cpu has
 >> the same cores/threads as your's and it sits idle 90% of the time.

 Ac> I have 8 threads/virtual cores, with 4 VMs and 2 virtual cores
 Ac> dedicated per VM. Is that not using up all the cores available to me?

This is where I think your getting things confused. The vm's are not set to run on a specific core.

They are given x number of cores they can use, and will only see that number. They could be running on core 1,3,5,6 and will
not know that. As far as the VM sees it's got access to 4 cores and that's what it will/can use.

A general estimation is that 1 vCPU = 1 Physical CPU Core. However, this is not entirely correct, as the vCPU is made up of
time slots across all available physical cores, so in general 1vCPU is actually more powerful than a single core, especially
if the physical CPUs have 8 cores.

As an example, I used to run 8 vm's on my server, with each vm doing pretty much only one thing.
I have now moved some of those task's into the other vm's, and have seen no change in the way the CPU performs.
What it has done is free up memory as I'm not running so many vm's.


A big of google searching:

vCPU vs. PCPU: A virtual CPU (vCPU) is a virtual processor allocated to a VM, while a physical CPU core (PCPU) is the actual
hardware processing unit. ESXi schedules vCPUs to run on available PCPU cores.

CPU Ready Time: This happens when a VM's vCPU is ready to run but must wait for a PCPU to become available, even if the
host's overall CPU usage isn't 100%. This can cause a VM to feel slow and is visible in tools like esxtop.

Hyperthreading: ESXi treats hyperthreads as separate CPUs, but a vCPU will still need a physical core to run.

Sockets = "physical" CPUs as the VM will interpret it.
Cores = cores on those physical sockets.
Ex: allocate 2 sockets with 2 cores - VM will report and see that it has two "physical" CPUs that are dual-core.

For the majority of what you will do with a VM, you will not need to allocate multiple sockets. Just cores. There is no
"hyperthreading" for the VM to take advantage of. The hypervisor allocated threads as "cores" for distribution.


Translated to VMs, one core = one processing thread.




Vorlon


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