Clarification on “Host” Meaning in FABRIC Testbed

[Fatih Berkay Sarpkaya]

Dear FABRIC Team,

I hope you are doing well. I have a question about the term “host” used in the FABRIC testbed.

When I reserve multiple VMs on the same host (for example, seat-w2.fabric-testbed.net), does this mean that all of these VMs are located on the same physical machine, or could they instead be distributed across different physical machines within the same rack or cluster?

If they could be on separate physical machines, how can I check whether my current slices are running on the same physical machine or not? Also, is there a way to guarantee co-location of multiple VMs on the same physical machine during reservation?

Thank you very much for your help and clarification.

Best regards,

Fatih Berkay Sarpkaya

[Komal Thareja]

Hi Fatih,

When requesting a VM in a slice, specifying the host parameter (for example, seat-w2.fabric-testbed.net) ensures that the VM is provisioned on that particular physical host. If multiple VMs in the same slice specify the same host (e.g., seat-w2.fabric-testbed.net), they will all be co-located on that same physical machine.

This can be done as follows:

slice.add_node(name="node1", host="seat-w2.fabric-testbed.net", ...)
slice.add_node(name="node2", host="seat-w2.fabric-testbed.net", ...)

If the host parameter is not specified, the FABRIC Orchestrator automatically places the VMs across available hosts based on resource availability, which may result in them being distributed across different physical machines.

If the requested host cannot accommodate the VMs (due to limited capacity or resource constraints), the system will return an “Insufficient resources” error.

Best regards,
Komal

[Fatih Berkay Sarpkaya]

Dear Komal,

Thank you so much for your quick reply. As I understand it, the term “host” refers to the same physical machine, and it is not distributed across multiple blades, correct?

For example, in the FABRIC resources, a single host is shown as having a total of 128 cores. Does this number represent 128 physical cores on one physical server? When I check inside a VM on that host, it reports: “AMD EPYC 7543 32-Core Processor “. This makes me think that the machine has 32 physical cores, which raises a question: is the “128 cores on the host” total coming from four separate machines (32 × 4)?

So, to confirm, you are saying that this interpretation is not correct, and that a “host” refers to one single physical machine. If so, could you please clarify that 128 cores and “AMD EPYC 7543 32-Core Processor ” difference?

Thank you very much for your clarification.

Best regards,

Fatih Berkay Sarpkaya

[Komal Thareja]

Hi Fatih,

You are absolutely correct — in the FABRIC testbed, the term “host” refers to a single physical machine, not a group of blades or multiple servers.

Regarding your question about the core count:
The host you mentioned (for example, seat-w2 ) reports 128 CPUs because the physical server has two AMD EPYC processors, each with 32 physical cores and hyperthreading — enabled. This means each physical core presents two logical CPUs (threads) to the operating system.

So, the breakdown is:

• 2 sockets × 32 physical cores per socket = 64 physical cores
• With hyperthreading (2 threads per core): 64 × 2 = 128 logical CPUs

Inside a VM, you’ll typically see the processor model name (e.g., AMD EPYC 7543 32-Core Processor), which corresponds to the physical CPU model installed in the host. The number of vCPUs visible in the VM depends on the resources allocated to it by the hypervisor, not the total physical core count of the host.

In summary:

• Host = one physical machine
• 128 cores = 64 physical cores × 2 threads (hyperthreading)
• VM CPU info = underlying processor model, showing only allocated vCPUs

You can find more details about the hardware configurations for a FABRIC site here:
https://learn.fabric-testbed.net/knowledge-base/fabric-site-hardware-configurations/

Best regards,
Komal

[Fatih Berkay Sarpkaya]

Dear Komal,

Thank you very much for your detailed explanation.  It was very helpful for me.

Best regards,

Fatih Berkay Sarpkaya

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