We setup 8 single servers to run the mongoimport test
import 50,000,000 documents each time.
these documents have been splitted to 32 parts and we run 4 mongoimport instances for each server.

At the beginning, the import speed is quite fast ( about 400 seconds each time ) when the mongod do not allocate the whole 64 GB RAM
But when the mongod has allocated all the RAM , there always have one or two server became take 99%+ CPU and import too slow.
These slow servers seem to randomly so that we can not figure out the reason.

And After 50 times import, we found some of these servers still seem to import very fast but others became too slow.
one import process will take 5000 seconds averagely or even more than 10000 seconds sometimes.

We had ran the same test for 1.6.5,1.7.5 and 1.8.0-rc0, and it's the same results.
We think the unstable performance is quite strange and maybe a problem of mongoimport or mongodb.