Glen Takahashi created SPARK-23249:
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Summary: Improve partition bin-filling algorithm to have less skew
and fewer partitions
Key: SPARK-23249
URL: https://issues.apache.org/jira/browse/SPARK-23249
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 2.2.1
Reporter: Glen Takahashi
Change DataSourceScanExec so that when grouping blocks together into
partitions, also checks the end of the sorted list of splits to more
efficiently fill out partitions.
h2. Rationale
The current bin-packing method of next-fit descending for blocks into
partitions is sub-optimal in a lot of cases and will result in extra
partitions, un-even distribution of block-counts across partitions, and un-even
distribution of partition sizes.
As an example, 128 files ranging from 1MB, 2MB,...127MB,128MB. will result in
82 partitions with the current algorithm, but only 64 using this algorithm.
Also in this example, the max # of blocks per partition in NFD is 13, while in
this algorithm is is 2.
More generally, running a simulation of 1000 runs using 128MB blocksize,
between 1-1000 normally distributed file sizes between 1-500Mb, you can see an
improvement of approx 5% reduction of partition counts, and a large reduction
in standard deviation of blocks per partition.
This algorithm also runs in O(n) time as NFD does, and in every case is
strictly better results than NFD.
Overall, the more even distribution of blocks across partitions and therefore
reduced partition counts should result in a small but significant performance
increase across the board
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