CPU performance and embedded systems

Three things that affect performance

• Clock speed: cycles per second, in hertz (Hz). 1 MHz (megahertz) is 1 000 000 Hz and 1 GHz (gigahertz) is 1 000 000 000 Hz, a thousand million, so 1 GHz = 1000 MHz. 3.2 GHz is 3 200 000 000 cycles a second. A faster clock means more instructions a second, but more heat. Running a CPU faster than its rated speed is overclocking: it gets hotter and can become unstable.
• Cores: each core is a separate processor. Several cores can work on different tasks at once, but a program only speeds up if its work can be split, and cores share memory and cache.
• Cache: the bigger the cache, the more often the CPU finds what it needs there instead of waiting for slower RAM.

time = number of cycles ÷ cycles per second

1. Time for one cycle = 1 ÷ clock speed. At 2 GHz: 1 ÷ 2 000 000 000 = 0.000 000 000 5 s = 0.5 nanoseconds (1 nanosecond is 10⁻⁹ s).
2. If each instruction takes 3 cycles, a 1.5 GHz CPU carries out 1 500 000 000 ÷ 3 = 500 000 000 instructions a second.
3. A task of 9 thousand million cycles on one core at 1.5 GHz takes 9 000 000 000 ÷ 1 500 000 000 = 6 seconds.
4. If the work splits perfectly between 3 cores, each does a third: 6 ÷ 3 = 2 seconds. To compare two CPUs, work out the time for each this way.
5. A job of 10 separate 4-second tasks on 2 cores, in the best case: each core does 5 tasks, so 5 × 4 = 20 seconds.