How do people measure performance overhead?
The meaning of overhead is usually an additional expense occurring in
addition to normal cost (whatever normal cost may be).

In the case of program, you may talk of overhead if it is run under
conditions such that it will cost more than it normally should, in
time or in memory usually, but it could be some other measure.

For example, at a time when compilers did not produce very good code,
compiled programs code size or execution time was compared with
programs handwritten in machine code. The quality of the compiler was
measured by the overhead of the generated code compared to a similar
program written in machine code. The overhead could be defined by the
ratio of the two costs, or by the percentage to be added. 

An overhead of 20% in code size meant that the compiled programhad a
machine code version 20% larger than the hanwritten one. An overhead
of 20% in speed meant that the compiled program took 20% more time to
execute. An overhead of 20% in memory meant that the compiled program
required 20% more memory to execute.

Nowadays, compilers are quite good and this kind of overhead would
probably be negative.  Compilers make the better code.

In the case of a virtual machine, you can have a program that run
directly on the hardware, or runs under the control of a virtual
machine that hides (and protect) somewhat the hardware. Running on a
virtual machine will slow the program, and the variation in running
time is the overhead (extra time cost) caused by the virtual machine.
You just measure the time taken by the same program running in both
situations, and you determine the percentage of additional time
required when running in a virtual machine.  It may depend on the type
of application you use to test the overhead of the virtual machine.

Overhead is a measure of extra cost. It applies to devices or programs
or procedure that cause an extra cost (but usually provide an extra
servive).

You could also measure the time overhead incurred by your programs
when you reduce the available real memory.

Overhead is a way of comparing two related performances, intended to
achive the same result, separately measured.

Does this clarify things for you?