Sergey Shelukhin created HIVE-20380:
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Summary: explore storing multiple CBs in a single cache buffer in
LLAP cache
Key: HIVE-20380
URL: https://issues.apache.org/jira/browse/HIVE-20380
Project: Hive
Issue Type: Bug
Reporter: Sergey Shelukhin
Lately ORC CBs are becoming ridiculously small. First there's the 4Kb minimum
(instead of 256Kb), then after we moved metadata cache off-heap, the index
streams that are all tiny take up a lot of CBs and waste space.
Wasted space can require larger cache and lead to cache OOMs on some workloads.
Reducing min.alloc solves this problem, but then there's a lot of heap (and
probably compute) overhead to track all these buffers. Arguably even the 4Kb
min.alloc is too small.
We should store contiguous CBs in the same buffer; to start, we can do it for
ROW_INDEX streams. That probably means reading all ROW_INDEX streams instead of
doing projection when we see that they are too small.
We need to investigate what the pattern is for ORC data blocks. One option is
to increase min.alloc and then consolidate multiple 4-8Kb CBs, but only for the
same stream. However larger min.alloc will result in wastage for really small
streams, so we can also consolidate multiple streams (potentially across
columns) if needed. This will result in some priority anomalies but they
probably ok.
Another consideration is making tracking less object oriented, in particular
passing around integer indexes instead of objects and storing state in giant
arrays somewhere (potentially with some optimizations for less common things),
instead of every buffers getting its own object.
cc [~gopalv] [~prasanth_j]
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