[jira] [Assigned] (HIVE-20380) explore storing multiple CBs in a single cache buffer in LLAP cache

["Sergey Shelukhin (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HIVE-20380?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Sergey Shelukhin reassigned HIVE-20380:
---------------------------------------

    Assignee: Sergey Shelukhin

> 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
>            Assignee: Sergey Shelukhin
>            Priority: Major
>
> 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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