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https://issues.apache.org/jira/browse/DRILL-5416?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15958012#comment-15958012
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Paul Rogers commented on DRILL-5416:
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The code here is complex. Since any change would be made only to improve a
memory estimate, this is not a good investment in time. Let's just live with
the incorrect memory estimates.
The good news is that the data, after being read, takes less memory than before
being read. (The vectors share a single "dead space" rather than a dead space
per vector.)
> Vectors read from disk report incorrect memory sizes
> ----------------------------------------------------
>
> Key: DRILL-5416
> URL: https://issues.apache.org/jira/browse/DRILL-5416
> Project: Apache Drill
> Issue Type: Bug
> Affects Versions: 1.8.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Minor
>
> The external sort and revised hash agg operators spill to disk using a vector
> serialization mechanism. This mechanism serializes each vector as a (length,
> bytes) pair.
> Before spilling, if we check the memory used for a vector (using the new
> {{RecordBatchSizer}} class), we learn of the actual memory consumed by the
> vector, including any unused space in the vector.
> If we spill the vector, then reread it, the reported storage size is wrong.
> On reading, the code allocates a buffer, based on the saved length, rounded
> up to the next power of two. Then, when building the vector, we "slice" the
> read buffer, setting the memory size to the data size.
> For example, suppose we save 20 1-byte fields. The size on disk is 20. The
> read buffer is rounded to 32 bytes (the size of the original, pre-spill
> buffer.) We read the 20 bytes and create a vector. Creating the vector
> reports the memory size as 20, "hiding" the extra, unused 12 bytes.
> As a result, when computing memory sizes, we receive incorrect numbers.
> Working with false numbers means that the code cannot safely operate within a
> memory budget, causing the user to receive an unexpected OOM error.
> As it turns out, the code path that does the slicing is used only for reads
> from disk. This ticket asks to remove the slicing step: just use the
> allocated buffer directly so that the after-read vector reports the correct
> memory usage; same as the before-spill vector.
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