Github user cloud-fan commented on the pull request:
https://github.com/apache/spark/pull/791#issuecomment-55356385
First, `ensureFreeSpace`(I renamed it to `findToBeDroppedBlocks`) doesn't
always return true. If it can't find enough to-be-dropped blocks to "free
space", it will return false.
Then if `ensureFreeSpace` return true, you can regard it as return a
`Future` that PROMISE you it will free a certain amount of memory after you
finish that `Future`. At that moment, the `freeMemory` is not updated(means the
memory has not been freed yet) until you finish the `Future`.
So if we put new blocks faster than the old blocks are dropped,
`ensureFreeSpace` will return false(because there is not enough memory), then
no to-be-dropped block is added, that thread will put that block into disk
store.
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