When FileAllocator worked, the whole of database seems to be locked during allocating physical data file.
The following log was written.

[FileAllocator] done allocating datafile /data/mongo/data/hoge1.5, size: 2047MB, took 21.081 secs

On this time, read/write operation was queued and timed out.
To avoid request timeout, we tried pre-preallocation by the following procedure.

1. creating database and collection at mongo console.
2. creating ".n" files and filling with zero.
3. creating large collection and confirm that FileAllocator did not create files and no request timeout occurred.

That is worked fine.
If possible, make FileAllocator free from locking.