Try 3.5 watts[1], not counting overhead of most USB power bricks being incredibly inefficient.
A current-gen 35W laptop CPU will be some 10 times faster[2] as a RasPi, have much faster storage available (SATA3 or NVMe versus… USB2), much faster I/O (GBit LAN and GBit Wifi versus… USB2), and a lot of other benefits. (Like an integrated screen and battery and keyboard and …) It also won't need external hardware to communicate with other cluster members – that 10-port ethernet switch will need power, too.
One RasPi is relatively energy efficient; RasPi clusters… not so much.
> But really, I think the primary use case is cost.
Indeed.
[1] http://raspi.tv/2016/how-much-power-does-raspberry-pi3b-use-... , see the numbers for "Multi-threaded CPU Tests", which is the most applicable for server workloads
[2] Running that script manages ~9 runs/second on an i7-6700HQ, vs. ~0.9 run/second on a RPi3.
And at $5 each, if we're talking hardware costs for setting up a "toy" cluster for, say, self-learning or student labs, that's hard to beat. I suppose you could do better using VMs for a virtual cluster, but that adds other complications unrelated to the clustering task. But I agree there doesn't otherwise seem to be much practical purpose here, and the overhead of running an OS on each Pi really cuts into performance compared to a single chip w/ multicores instead.
[1] https://www.jeffgeerling.com/blogs/jeff-geerling/raspberry-p...
At the same time, you're comparing the power consumption of lets say 10 whole RPis/platforms to the consumption of a single processor. Stick that processor in a platform (laptop), and it's going to use much more than 40W.
Like you said, you get a lot more with the laptop, but given your benchmark (10x difference), my guess is that 10x RPis would still be more power efficient than a laptop with a 6700HQ at that specific task.
http://ark.intel.com/products/88967/Intel-Core-i7-6700HQ-Pro...
Gets you 11 Pi's ... Gets you only 1 Intel CPU, no memory, motherboard, heatsink, fans.
Reminds me of the Celeron® Processor J3455... 10W rating on Intel there page. On AVERAGE! Then when you see the real power usage under load for MB + CPU + 16GB memory, its actually doing 35W.
Where as the Pi's are doing 3.7W max per piece. So even with 4 pieces to match the performance, your still half the wattage.
If Intel really scaled that good in power vs performance, why are we not seeing x86 phones all the time?
But really, I think the primary use case is cost. Actually having access to that many physical machines to play with in a classroom or home learning environment is sort of new! The market hasn't really had such accessible linux computers at "Ehh, if it breaks I'll just buy a new one, no big deal" prices. It's educational, and the more stable the ARM support is, the better a student's skills will transfer over into the real world of systems administration.