Scaled Speedup
Time-constrained Speedup--as the size of the machine increases, the problem size is increased to meet the fixed execution time.
Memory-constrained Speedup--the greatest problem size that can be solved in a system.
A system is scalable iff it can achieve the ideal scaled (fixed-time or memory-bound speedup).
Problem size-constrained Speedup--how fast can a given problem be solved?