Mining is taking too much cpu memory( i.e more than 100% of one core), how to reduce it?

Question

What I have
1. I have step up four private blockchain of ethereum.
2. when i start mining on one core( miner.start(1) ) , it is taking too much cpu memory( i.e 100% or more)

What I Want
1. Mining should take less memory
2. I have to run four instance of geth on a system, connected to each other( in a peer)