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When we are talking about "1 vcpu" in different cloud solutions:

Q: What exactly does it mean?

Ex.: if I will use a 10 vcpu machine for the same cpu intensive process (which only uses CPU) then I would see the same amount of CPU load on them? Would they finish at the same time regarding the same only-CPU-usage task?

Or the definition of vcpu could differ in the different cloud solutions?

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vcpu = Virtual CPU, that's just the number of core of the instance. Nothing more, nothing less.

For the rest of your questions they're unrelated, this depends on the underlying hypervisor's physical processors, there's no guarantee they use the same type with same frequency.

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The definition can definitely vary, for example AWS defines vCPU as:

AWS: Each vCPU is a thread of a CPU core, except for ...

Similarly, Azure and Google also tend to match a vCPU to a hyper-thread rather than always 1:1 with an actual core.

Google: For the n1 series of machine types, a vCPU is implemented as a single hardware hyper-thread on one of the available CPU Platforms.

Generally, vCPU is used as a metric for simple comparison of machines within a vendor's range of machines.

It may also used a cost equivalence factor (e.g. within the same machine type family, a 4 vCPU machine would be twice the cost of 2 vCPU machine)

How well a certain load can fully utilise a machine's resources, and how performance between different machine families and vendors, depends on multiple factors. So if it's important to know for sure then it would be best to run some real tests; otherwise you can take the vCPU count as an approximate indicator.

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