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From "Wei Zheng (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-12837) Better memory estimation/allocation for hybrid grace hash join during hash table loading
Date Mon, 11 Jan 2016 21:55:39 GMT

    [ https://issues.apache.org/jira/browse/HIVE-12837?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15092757#comment-15092757
] 

Wei Zheng commented on HIVE-12837:
----------------------------------

Yes, general cases are also handled, in isMemoryFull check. Every time when I check memory,
I want to make sure there are enough space to create one write buffer for each partition,
otherwise I will spill. Sounds right?

> Better memory estimation/allocation for hybrid grace hash join during hash table loading
> ----------------------------------------------------------------------------------------
>
>                 Key: HIVE-12837
>                 URL: https://issues.apache.org/jira/browse/HIVE-12837
>             Project: Hive
>          Issue Type: Bug
>          Components: Hive
>    Affects Versions: 2.1.0
>            Reporter: Wei Zheng
>            Assignee: Wei Zheng
>         Attachments: HIVE-12837.1.patch
>
>
> This is to avoid an edge case when the memory available is very little (less than a single
write buffer size), and we start loading the hash table. Since the write buffer is lazily
allocated, we will easily run out of memory before even checking if we should spill any hash
partition.
> e.g.
> Total memory available: 210 MB
> Size of ref array of BytesBytesMultiHashMap for each hash partition: ~16 MB
> Size of write buffer: 8 MB (lazy allocation)
> Number of hash partitions: 16
> Number of hash partitions created in memory: 13
> Number of hash partitions created on disk: 3
> Available memory left after HybridHashTableContainer initialization: 210-16*13=2MB
> Now let's say a row is to be loaded into a hash partition in memory, it will try to allocate
an 8MB write buffer for it, but we only have 2MB, thus OOM.
> Solution is to perform the check for possible spilling earlier so we can spill partitions
if memory is about to be full, to avoid OOM.



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