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https://issues.apache.org/jira/browse/HIVE-20380?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sergey Shelukhin updated HIVE-20380:
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Description:
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.
The initial idea was to store multiple CBs per block, however this is a lot of
work all over the place (cache mapping, cache lookups, readers, etc.).
There are also minor heap improvements possible.
1) Intern tracking tag.
2) Replace AtomicLong object with a long and unsafe CAS method, we only ever
use one method, compareAndSwap.
One more idea 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]
was:
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]
> LLAP cache should cache small buffers more efficiently
> ------------------------------------------------------
>
> 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.
> The initial idea was to store multiple CBs per block, however this is a lot
> of work all over the place (cache mapping, cache lookups, readers, etc.).
> There are also minor heap improvements possible.
> 1) Intern tracking tag.
> 2) Replace AtomicLong object with a long and unsafe CAS method, we only ever
> use one method, compareAndSwap.
> One more idea 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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