[jira] [Created] (SPARK-13695) Don't cache blocks at different serialization level when reading back from disk

["Josh Rosen (JIRA)" <ji...@apache.org>]

Josh Rosen created SPARK-13695:
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             Summary: Don't cache blocks at different serialization level when reading back from disk
                 Key: SPARK-13695
                 URL: https://issues.apache.org/jira/browse/SPARK-13695
             Project: Spark
          Issue Type: Improvement
          Components: Block Manager
            Reporter: Josh Rosen
            Assignee: Josh Rosen


When a cached block is spilled to disk and read back in serialized form (i.e. as bytes), the current BlockManager implementation will attempt to re-insert the serialized block into the MemoryStore even if the block's storage level requests deserialized caching.

This behavior adds some complexity to the MemoryStore but I don't think it offers many performance benefits and I'd like to remove it in order to simplify a larger refactoring patch. Therefore, I propose to change the behavior such that disk store reads will only cache bytes in the memory store for blocks with serialized storage levels.

There are two places where we request serialized bytes from the BlockStore:

1. getLocalBytes(), which is only called when reading local copies of TorrentBroadcast pieces. Broadcast pieces are always cached using a serialized storage level, so this won't lead to a mismatch in serialization forms if spilled bytes read from disk are cached as bytes in the memory store.

2. the non-shuffle-block branch in getBlockData(), which is only called by the NettyBlockRpcServer when responding to requests to read remote blocks. Caching the serialized bytes in memory will only benefit us if those cached bytes are read before they're evicted and the likelihood of that happening seems low since the frequency of remote reads of non-broadcast cached blocks seems very low. Caching these bytes when they have a low probability of being read is bad if it risks the eviction of blocks which are cached in their expected serialized/deserialized forms, since those blocks seem more likely to be read in local computation.

Therefore, I think this is a safe change.



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