showing 8 cores max on 16 core WS2019 Server

[alexanderz2000]

Hello all,

I'm new to to this community but need some help with with workstation pro 17.5, I downloaded the trial to test it out before I purchase the software to get things up and running. However, my only issue is that application that the VM is hosted has 2 processors 'HPDL360 Gen9' with 8 cores each, 16 cores total, when i configure any other clients, W10, W11, Server 2012 and Server 2019, the software will not let me configure all 16 cores regardless of the configuration combination i try, it always says VM will fail because it's configured to use more Virtual processor cores than the host supports.   That can't be true, my server has 2 processors with 8 cores each = 16 total, any know whats going on?  

[alexanderz2000]

OK, solved my own question but nowhere to be found on the net, we all know Virtualization needs to be enabled in the BIOS but it's not stated that 'Hyper Threading' also needs to be enabled.  Once I enabled it, I was able to use all my cores on each client.   Just FYI for someone that comes across this same issue. 

[Mikero · Accepted solution]

FWIW, you can only assign as many cores as are available on a single CPU.

Without hyperthreading, in your case, that means 8.

With HT, you now have 16, but not 32.

[alexanderz2000]

Correct, each of my processors only support 8 cores so 16 would be max cores on each client, I'm not sure where 32 came from but it's unfortunate that the documentation mentions nothing requiring HT to enable all cores which could have saved me weeks of frustration.  At least is all working as intended so happy about that.  

[Technogeezer]

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@Mikero wrote:

FWIW, you can only assign as many cores as are available on a single CPU.

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@Mikero , what's the rationale behind not allowing more cores  than are present on a single CPU socket to be assigned to a VM? Seems like an arbitrary limitation if you have a multi-CPU system.

[Mikero]

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@alexanderz2000 wrote:

[...] I'm not sure where 32 came from [...] 

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I was saying you can't add the cores between two CPUs in the VM config.

Re: HT... We make some assumptions about desktop configurations, it's assumed that HT is always on by default.

[Mikero]

I'm not sure at the lowest of levels why that is, but I suspect it has to do with scheduling complications between physical CPUs. Memory is attached to specific CPUs, so threading gets expensive/problematic when trying to run a thread with memory attached to the other CPU socket.

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@Technogeezer wrote:

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@Mikero wrote:

FWIW, you can only assign as many cores as are available on a single CPU.

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@Mikero , what's the rationale behind not allowing more cores  than are present on a single CPU socket to be assigned to a VM? Seems like an arbitrary limitation if you have a multi-CPU system.

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[bluefirestorm]

I am able to create/run VMs with 32vCPUs on a Windows 10 host with 2x E5-2680 v4 CPU (14c/28t); total 56 logical CPUs with HT enabled; so it can also run across NUMA nodes. If HT is disabled at the host UEFI, the total logical CPUs is 28; so maximum a VM can be configured is 24 vCPUs; but that is still across NUMA nodes.

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alexanderz2000 wrote:

Correct, each of my processors only support 8 cores so 16 would be max cores on each client, I'm not sure where 32 came from but it's unfortunate that the documentation mentions nothing requiring HT to enable all cores which could have saved me weeks of frustration.

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If the CPU has 8 cores with HT enabled, that should be 16 logical processors. With 2 CPU sockets, that should bring up the number to 32. So rightfully, in your DL360 Gen9, a VM can have 24 vCPUs or even 32 vCPUs, although it is better not to max out on a Windows host.

So there must be something else that is wrong with the host OS.

Run msconfig
Check the "Boot" tab - Advanced Options button and the Processors and make sure there is no limit placed.

The dropdown list box should contain 32 in your case; any number lower than likely means that something else is wrong with the host OS (perhaps licensing on the Server 2019 OS ???).