MapStatus — Shuffle Map Output Status

There are two types of MapStatus:

CompressedMapStatus that compresses the estimated map output size to 8 bits (Byte) for efficient reporting.
HighlyCompressedMapStatus that stores the average size of non-empty blocks, and a compressed bitmap for tracking which blocks are empty.

When the number of blocks (the size of uncompressedSizes) is greater than 2000, HighlyCompressedMapStatus is chosen.

Caution

FIXME What exactly is 2000? Is this the number of tasks in a job?

trait MapStatus {
  def location: BlockManagerId
  def getSizeForBlock(reduceId: Int): Long
}

Note	`MapStatus` is a `private[spark]` contract.

Table 1. `MapStatus` Contract
Method	Description
`location`	The BlockManager where a `ShuffleMapTask` ran and the result is stored.
`getSizeForBlock`	The estimated size for the reduce block (in bytes).

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