Power efficiency

[Dirk Broer]

A while ago one of the postings on our forum read "hot=inefficient" and while the hot in this equation is wasted heat, one might ask oneself whether a 'hot' CPU is actually less efficient than a cool one, when efficiency is measured as to how much of the wattage spend goes into computing. I have compared all recent CPUs from both blue and green and there are some surprising results. How did I do this? I took for each CPU the number of threads that the CPU can spend on BOINC CPU projects and divided that by the TDP value for that CPU, so you can see how many Watts go into a single thread. Are AMD FX CPUs hot and therefore inefficient? Are i5s the total kings of cool? Not quite, and though Intel takes four of the best scores, we encounter the first -95 Watt- Bulldozers and Piledrivers on a shared 7th spot:

Ranking	Brand	Type	Socket	Model	Threads	TDP	Score
1	Intel	i7	1155	IvyBridge	8	77	9.63
2	Intel	i7	1150	Haswell	8	84	10.50
3	Intel	i7	2011	Ivy Bridge-E	12	130	10.83
3	Intel	i7	2011	Sandy Bridge-E	12	130	10.83
3	Intel	i7	1366	Gulftown	12	130	10.83
6	AMD	A8, A10	FM2	Richland T	4	45	11.25
7	AMD	FX	AM3+	Vishera	8	95	11.88
7	AMD	FX	AM3+	Zambezi	8	95	11.88
7	Intel	i7	1155	SandyBridge	8	95	11.88
7	Intel	i7	1156	Lynnfield	8	95	11.88
11	Intel	i7	2011	SandyBridge-EX	12	150	12.50
12	Intel	i3	1150	Haswell	4	54	13.50
13	Intel	i3	1155	IvyBridge	4	55	13.75
14	AMD	FX	AM3+	Vishera	8	125	15.63
15	AMD	FX	AM3+	Zambezi	8	125	15.63
16	AMD	FX	AM3+	Vishera	6	95	15.83
16	AMD	FX	AM3+	Zambezi	6	95	15.83
16	AMD	Phenom II X6	AM3	Thuban	6	95	15.83

The shared 19th place sees a lot of 65 Watt quads, amongst them the most efficient AMD APUs. The reknowned Intel i5 does not make the cut as it needs too much Watts to feed it's four threads, no matter what socket it is combined with. The Intel Pentium G (incarnation Clarkdale, Socket 1156) is amongst the worst performers, its mere 77 Watt is needed for just two threads. Even worse is AMDs Phenom II X2 Callisto: 80 Watt for just two threads.