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Posted to github@arrow.apache.org by "alamb (via GitHub)" <gi...@apache.org> on 2023/07/24 10:48:48 UTC
[GitHub] [arrow-datafusion] alamb opened a new issue, #7065: Improve aggregate performance with adaptive sizing in accumulators / avoiding reallocations in accumulators
alamb opened a new issue, #7065:
URL: https://github.com/apache/arrow-datafusion/issues/7065
### Is your feature request related to a problem or challenge?
Making aggregations fast in datafusion helps its adoption and makes it even cooler
As part of the new #6904 work, @yjshen had an idea https://github.com/apache/arrow-datafusion/pull/6800#discussion_r1251142165 that could avoid a copy in the accumulator implementations:
### Describe the solution you'd like
```Adaptive sizing(perhaps?): How would the hash table header and states in each accumulator initialize and grow their sizes afterward?```
Here is the structure of the current group operator
```
┌──────────────┐ ┌──────────────┐ ┌──────────────┐
│┌────────────┐│ │┌────────────┐│ │┌────────────┐│
┌─────┐ ││accumulator ││ ││accumulator ││ ││accumulator ││
│ 5 │ ││ 0 ││ ││ 0 ││ ││ 0 ││
├─────┤ ││ ┌────────┐ ││ ││ ┌────────┐ ││ ││ ┌────────┐ ││
│ 9 │ ││ │ state │ ││ ││ │ state │ ││ ││ │ state │ ││
├─────┤ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
│ │ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
├─────┤ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
│ 1 │ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
├─────┤ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
│ │ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
└─────┘ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
││ │ │ ││ ││ │ │ ││ ││ │ │ ││
││ └────────┘ ││ ││ └────────┘ ││ ││ └────────┘ ││
│└────────────┘│ │└────────────┘│ │└────────────┘│
Hash Table └──────────────┘ └──────────────┘ └──────────────┘
New NewAccumulator
stores "group indexes" There is one GroupsAccumulator per aggregate
which are indexes into (NOT PER GROUP). Internally, each
Vec<GroupState> GroupsAccumulator manages the state for
multiple groups
```
The implementation of this, such as Average (TODO link) is to use a `Vec<T>`. While this approach is simple to implement, it also means that as the Vec grows, the accumulated vales may be copied (up to 2 times on average, given a doubling strategy)
An alternate, suggested by @yjshen is to segment the state into **fixed-sized vectors, allocate a new vector at a time**, fill it until full, then create a new vector for upcoming new states.
```
┌──────────────┐ ┌──────────────┐ ┌──────────────┐
│┌────────────┐│ │┌────────────┐│ │┌────────────┐│
┌─────┐ ││accumulator ││ ││accumulator ││ ││accumulator ││
│ 5 │ ││ AGG ││ ││ SUM ││ ││ 0 ││
├─────┤ ││ ┌────────┐ ││ ││ ┌────────┐ ││ ││ ┌────────┐ ││
│ 9 │ ││ │ state- │ ││ ││ │ state- │ ││ ││ │ state │ ││
├─────┤ ││ │segment-│ ││ ││ │segment-│ ││ ││ │ │ ││
│ │ ││ │ 1 │ ││ ││ │ 1 │ ││ ││ │ │ ││
├─────┤ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
│ 1 │ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
├─────┤ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
│ │ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
└─────┘ ││ │ │ ││ ││ │ │ ││ ││ │ │ ││
││ │ │ ││ ││ │ │ ││ ││ │ │ ││
││ └────────┘ ││ ││ └────────┘ ││ ││ └────────┘ ││
││ ││ ││ ││ │└────────────┘│
Hash Table ││ ┌────────┐ ││ ││ ┌────────┐ ││ └──────────────┘
││ │ state- │ ││ ││ │ state- │ ││
││ │segment-│ ││ ││ │segment-│ ││
││ │ 2 │ ││ ││ │ 2 │ ││
││ │ │ ││ ││ │ │ ││
││ │ │ ││ ││ │ │ ││
││ │ │ ││ ││ │ │ ││
││ │ │ ││ ││ │ │ ││
││ │ │ ││ ││ │ │ ││
││ │ │ ││ ││ │ │ ││
││ └────────┘ ││ ││ └────────┘ ││
│└────────────┘│ │└────────────┘│
└──────────────┘ └──────────────┘
```
Thru this segmented approach, we could avoid memory copy for each resize, which the number of resizing would be great for high cardinality aggs, and grows the size more predictably.
But admittedly, this approach would also bring complexity for both header pointer management and update span multiple vectors.
### Describe alternatives you've considered
_No response_
### Additional context
_No response_
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Re: [I] Improve aggregate performance with adaptive sizing in accumulators / avoiding reallocations in accumulators [arrow-datafusion]
Posted by "alamb (via GitHub)" <gi...@apache.org>.
alamb commented on issue #7065:
URL: https://github.com/apache/arrow-datafusion/issues/7065#issuecomment-1830067924
> But what does adaptive mean in with adaptive sizing?
I think @yjshen meant something like storing the values in `Vec<Vec<T>>` rather than `Vec<T>`
And when more space was needed, create a new `Vec::with_capacity` rather than push to the existing `Vec` (which might reallocate and copy)
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Re: [I] Improve aggregate performance with adaptive sizing in accumulators / avoiding reallocations in accumulators [arrow-datafusion]
Posted by "my-vegetable-has-exploded (via GitHub)" <gi...@apache.org>.
my-vegetable-has-exploded commented on issue #7065:
URL: https://github.com/apache/arrow-datafusion/issues/7065#issuecomment-1827953667
It seems like deque in cpp ？
But what does adaptive mean in ```with adaptive sizing```?
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