VCPU is the other half of hyperthreading/smt

Rhe route I have gone is a used cpu board combination there are tons of
them on ebay and they are a single epyc but combined less than 300. And the
core density is amazing.

The again.
NEW  AMD Epyc 7551 32-Core 64-Thread 2.0GHz CPU Retail Box NO VENDOR Lock
for 99 bux. That's worth it.

On Thu, Jul 20, 2023, 5:17 PM Ryan Petris via PLUG-discuss <
plug-discuss@lists.phxlinux.org> wrote:

> That's right, a vCPU is a shared resource.
>
> Lets say that you had a machine with 4 cores on it. To the operating
> system, other than some nuances with the cache, PCI connections, etc., each
> core is effectively its own CPU, and therefore we'll say the machine has a
> total of 4 CPUs. Now, just becuase it has 4 CPUs doesn't mean that you can
> only run 4 programs at a time; you can run hundreds or thousands of
> programs at the time time, and the operating system will schedule those to
> run on the CPUs. Each program will get some CPU time, more so if other
> processes aren't doing much.
>
> Now, think of each vCPU as a process running on the host machine. The host
> machine will time share those vCPUs across the actual CPUs just like any
> other process running on the machine. But, within the virtual environment,
> it is seen as a real CPU to the guest operating system, and it will
> schedule programs to run on that CPU accordingly.
>
> So, you effectively have two layers of scheduling happening.
>
> There's not necessarily a limit on the number of vCPUs, just as there's
> not necessarily a limit on the number of processes that you can run. Yes
> there are technical limits on the number of processes that can be run, but
> that's an operating system limitation.
>
> Now, you can assign a vCPU to have exclusive use of a CPU on the host
> system, however unless you have some critical application where that's an
> absolute must, it's really not necessary; you're just slowing down every
> other process on that machine as they're no longer able to use that CPU
> when nothing else is using it.
>
> Now, if your machine is super busy and you have virtual machines competing
> for time, then you have what's called "steal time" or "CPU Steal", which is
> the amount of time a vCPU waits for control of a real CPU; you can read
> about that here:
> https://blog.appsignal.com/2021/09/15/cpu-steal-time-a-crucial-metric-for-cloud-servers-and-vms.html.
> But if that becomes a problem, you're overloading your system and it
> doesn't really matter how many threads your host system has.
>
> On Thu, Jul 20, 2023, at 12:59 PM, techlists@phpcoderusa.com wrote:
>
> On 2023-07-20 10:55, Ryan Petris wrote:
> > The CPU's cheap because it's old and no one wants them anymore -- it's
> > of the same generation as 6000 series intel processors (i.e. skylake).
> > It also uses a server socket, so the only motherboards you're going to
> > be able to find are server motherboards. Those are going to be
> > expensive and/or have other quirks, such as requiring a vendor
> > specific heatsink, or a vendor-specific power supply, or take 5
> > minutes to start up, etc.
> >
> > You'd be better off spending money on a last-gen cpu and motherboard,
> > for instance here's a combination that is relatively cheap:
> >
> > $174 for an i5-12400, which according to cpubenchmark.net is nearly
> > 30% faster than the Xeon you linked (score of 19501 vs 15146, much
> > faster single-core score as well):
> >
> https://www.amazon.com/Intel-i5-12400-Desktop-Processor-Cache/dp/B09NMPD8V2/
> >
> > $139 for a compatible motherboard:
> > https://www.amazon.com/GIGABYTE-B760M-DS3H-AX-Motherboard/dp/B0BSP61QZC/
> >
> > I also wouldn't pay so much attention to the number of "threads" you
> > think you'll need; you can run many VMs with a total number of virtual
> > processors that is much more than what you actually have, and as long
> > as you're not trying to go whole hog on every machine at the same time
> > you'll be fine, and even if you do, you'll still be better off with a
> > faster processor with a few fewer threads than an older slower cpu
> > with more.
>
> ---
>
> Several months ago I did some research on what a vCPU is.  I could not
> find an exact answer.  What I came away thinking was a vCPU is equal to
> a thread.  From what you are saying it sounds like a vCPU is a shared
> resource, so there may be more vCPUs than actual threads?  Is there a
> way I can determine the number of vCPUs a CPU will provide?
>
>
> ---
>
> >
> > On Thu, Jul 20, 2023, at 10:26 AM, Keith Smith via PLUG-discuss wrote:
> >
> >> Hi,
> >>
> >> I was surfing the Inter Web when I happened upon a Xeon server CPU.
> >> It
> >>
> >> is marked at $32.49 at Newegg.  It has 12 cores and 24 threads and
> >> has a
> >>
> >> good benchmark score.
> >>
> >>
> >
> https://www.cpubenchmark.net/cpu.php?cpu=Intel+Xeon+Silver+4116+%40+2.10GHz&id=3179
> >>
> >> https://www.newegg.com/p/274-000A-007K2?Description=Xeon
> >>
> >> In the future at some point I would like to build something with 20
> >> plus
> >>
> >> or minus cores and 40 threads more or less for Proxmox.  This would
> >> be
> >>
> >> over kills because I only need 1 or 2 VMs active at one time...
> >> maybe 3
> >>
> >> in an extreme situation.
> >>
> >> This 12 core/24 thread CPU with 64Gb of Ram and a 1Tb SSD would
> >> really
> >>
> >> be more resources than I would ever need.  Off the top of my head
> >> this
> >>
> >> means I might be able to build a decent Proxmox server for $500 -
> >> $600.
> >>
> >> I do not need fancy video except for one VM that might be running
> >> Win 10
> >>
> >> or 11...  I assume a server grade CPU would handle Win 10 and 11?
> >>
> >> Am I on the right track?
> >>
> >> Thank You For Your Feedback!!
> >>
> >> Keith
> >>
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> >>
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