EdisonWang created SPARK-37593:
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Summary: Optimize HeapMmeoryAllocator to avoid memory waste in
humongous allocation when using G1GC
Key: SPARK-37593
URL: https://issues.apache.org/jira/browse/SPARK-37593
Project: Spark
Issue Type: Improvement
Components: Spark Core, SQL
Affects Versions: 3.3.0
Reporter: EdisonWang
Fix For: 3.3.0
As we may know, a phenomenon called humongous allocations exists in G1GC when
allocations that are larger than 50% of the region size.
Spark's tungsten memory model usually tries to allocate memory by one `page`
each time and allocated by long[pageSizeBytes/8] in
HeapMemoryAllocator.allocate.
Remember that java long array needs extra object header (usually 16 bytes in
64bit system), so the really bytes allocated is pageSize+16.
Assume that the G1HeapRegionSize is 4M and pageSizeBytes is 4M as well. Since
every time we need to allocate 4M+16byte memory, so two regions are used with
one region only occupies 16byte. Then there are about 50% memory waste.
It can happenes under different combinations of G1HeapRegionSize (varies from
1M to 32M) and pageSizeBytes (varies from 1M to 64M).
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