Poorly.
Do you mean a Beowulf cluster of mobile phones? It seems like it would be woefully inefficient to say the least.
- each node in the cluster has an expensive screen attached to it, which would presumably be permanently disabled to reduce cooling/power costs. This is a waste of money. Same goes for bluetooth, accelerometers and anything else that is not directly contributing to the cluster. The cost of the screen etc. could have purchased more processors instead.
- temperature management would be incredibly inefficient. How would you cool a rack of 1000 phones/tablets? Server chassis are designed with this in mind. Phones/tablets are not.
It's an interesting idea to think about, but nobody is going to produce a cluster powered by iPhone 6s which is capable of competing (financially or computationally) with a cluster powered by servers designed for this purpose.
Are you suggesting that the economy of scale has reached a point where the smartphone would be more cost-effective even with those features?
This is a futuristic idea - think of the 80s and the 8086 has just come out, but one day you're going to challenge the mainframes. How do we make a household capable of being an Amazon
Put a sysadmin in every household. Personal servers have been a thing in geek houses for decades.
We're starting to get more domestic computer appliances: PVRs, consoles, home NAS gizmos, increasingly smart routers. Nest. Smart lightbulbs. However, because very few people have the skill, time, and inclination to manage these things, they're subject to the management of their OEM. Who will do the profit-maximising thing.
Can the millions of phones be used to coordinate and build any sort of 'Cloud servers'? When SETI@Home used our desktops' idle screen saver time for its calculations, can these multi-core pocket computers be used for something?
To encourage people to contribute processing power / storage, these projects need to incentivise them which is where bitcoin comes in.
There is a startup called 21.co which is creating an embeddable bitcoin mining chip, which can be "embedded into an internet-connected device as a standalone chip or integrated into an existing chipset as a block of IP to generate a continuous stream of digital currency for use in a wide variety of applications." [1]
As mobile devices start having these embeddable mining chips, people would be able to "mine" coins like FoldingCoin and CureCoin on their devices, which then becomes like a network of coordinated "cloud servers" which are incentivised by these coins
Also, there are decentralised cloud storage projects like Storj.io (similar to Dropbox).
[1] https://medium.com/@21dotco/a-bitcoin-miner-in-every-device-...
