GitHub user liancheng opened a pull request:
https://github.com/apache/spark/pull/15976
[SPARK-18403][SQL] Fix unsafe data false sharing issue in
ObjectHashAggregateExec
[SPARK-18403][SQL] Fix unsafe data false sharing issue in
ObjectHashAggregateExec
## What changes were proposed in this pull request?
This PR fixes a random OOM issue occurred while running
`ObjectHashAggregateSuite`.
This issue can be steadily reproduced under the following conditions:
1. The aggregation must be evaluated using `ObjectHashAggregateExec`;
2. There must be an input column whose data type involves `ArrayType` (an
input column of `MapType` may even cause SIGSEGV);
3. Sort-based aggregation fallback must be triggered during evaluation.
The root cause is that while falling back to sort-based aggregation, we
must sort and feed already evaluated partial aggregation buffers living in the
hash map to the sort-based aggregator using an external sorter. However, the
underlying mutable byte buffer of `UnsafeRow`s produced by the iterator of the
external sorter is reused and may get overwritten when the iterator steps
forward. After the last entry is consumed, the byte buffer points to a block of
uninitialized memory filled by `5a`. Therefore, while reading an
`UnsafeArrayData` out of the `UnsafeRow`, `5a5a5a5a` is treated as array size
and triggers a memory allocation for a ridiculously large array and immediately
blows up the JVM with an OOM.
To fix this issue, we only need to add `.copy()` accordingly.
## How was this patch tested?
New regression test case added in `ObjectHashAggregateSuite`.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/liancheng/spark investigate-oom
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/15976.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #15976
----
commit 4b88eed0f5f2cb34d217d833dbe085f9d0277a94
Author: Cheng Lian <[email protected]>
Date: 2016-11-21T23:41:23Z
Fix unsafe data false sharing issue in ObjectHashAggregateExec
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