A generic parallel computer consists of a number of processors connected together. The machine may be very expensive or very cheap. I have deliberately not written examples here. It may not be cost effective to use an expensive parallel machine with a fast interconnection network to perform independent simulations on each node if there a cluster of machines available which will perform the same task. Not necessarily a factor of P on P processors.
P is obtained on P processors.
The speedup should be measured wrt to the best sequential algorithm on a single node. The aggregate memory on several nodes may allow a different algorithm. Increased cache on several nodes may allow more of the code and instructions to run in core. Speedup is of academic interest. It is possible to obtain perfect speedup curves with a poor parallel algorithm if the single node time is slow.