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https://issues.apache.org/jira/browse/SPARK-13695?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Apache Spark reassigned SPARK-13695:
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Assignee: Josh Rosen (was: Apache Spark)
> Don't cache MEMORY_AND_DISK blocks as bytes in memory store when reading
> spills
> -------------------------------------------------------------------------------
>
> 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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