If we observe the performance benchmark numbers across a number of runs, even though the hardware, load, OS, and so on remain the same, the numbers are rarely exactly the same. The difference between each run may be upto as much as ±8 percent for no apparent reason. This may seem surprising, but the deep-rooted reason is that the performances of computer systems are stochastic by nature. There are many small factors in a computer system that make performance unpredictable at any given point of time. At best, the performance variations can be explained by a series of probabilities over random variables.
The basic premise is that each subsystem is more or less like a queue where requests await their turn to be served. The CPU has an instruction queue with unpredictable fetch/decode/branch-predict timings; the memory access again depends on the cache hit ratio and whether it needs to be dispatched via the interconnect, I/O subsystem works using interrupts that...