Scalability is the degree to which workload throughput benefits from the availability of additional processors. It is usually expressed as the quotient of the throughput of the workload on a multiprocessor divided by the throughput on a comparable uniprocessor. For example, if a uniprocessor achieved 20 requests per second on a given workload and a four-processor system achieved 58 requests per second, the scaling factor would be 2.9. That workload is highly scalable. Most real-world workloads would not.
Scalability is very difficult to estimate.
Whenever possible, scalability assumptions should be based on measurements of authentic workloads. The workload consists of a series of commands. On the uniprocessor, only one command can actually be processing at a time, regardless of whether or not the lock is held. On the multiprocessor, there are two processors to handle program execution, but there is still only one lock. In the period shown, the multiprocessor handles 14 commands.