# Objective
- fix#12853
- Make `Table::allocate` faster
## Solution
The PR consists of multiple steps:
1) For the component data: create a new data-structure that's similar to
`BlobVec` but doesn't store `len` & `capacity` inside of it: "BlobArray"
(name suggestions welcome)
2) For the `Tick` data: create a new data-structure that's similar to
`ThinSlicePtr` but supports dynamic reallocation: "ThinArrayPtr" (name
suggestions welcome)
3) Create a new data-structure that's very similar to `Column` that
doesn't store `len` & `capacity` inside of it: "ThinColumn"
4) Adjust the `Table` implementation to use `ThinColumn` instead of
`Column`
The result is that only one set of `len` & `capacity` is stored in
`Table`, in `Table::entities`
### Notes Regarding Performance
Apart from shaving off some excess memory in `Table`, the changes have
also brought noteworthy performance improvements:
The previous implementation relied on `Vec::reserve` &
`BlobVec::reserve`, but that redundantly repeated the same if statement
(`capacity` == `len`). Now that check could be made at the `Table` level
because the capacity and length of all the columns are synchronized;
saving N branches per allocation. The result is a respectable
performance improvement per every `Table::reserve` (and subsequently
`Table::allocate`) call.
I'm hesitant to give exact numbers because I don't have a lot of
experience in profiling and benchmarking, but these are the results I
got so far:
*`add_remove_big/table` benchmark after the implementation:*
*`add_remove_big/table` benchmark in main branch (measured in comparison
to the implementation):*
*`add_remove_very_big/table` benchmark after the implementation:*
*`add_remove_very_big/table` benchmark in main branch (measured in
comparison to the implementation):*
cc @james7132 to verify
---
## Changelog
- New data-structure that's similar to `BlobVec` but doesn't store `len`
& `capacity` inside of it: `BlobArray`
- New data-structure that's similar to `ThinSlicePtr` but supports
dynamic allocation:`ThinArrayPtr`
- New data-structure that's very similar to `Column` that doesn't store
`len` & `capacity` inside of it: `ThinColumn`
- Adjust the `Table` implementation to use `ThinColumn` instead of
`Column`
- New benchmark: `add_remove_very_big` to benchmark the performance of
spawning a lot of entities with a lot of components (15) each
## Migration Guide
`Table` now uses `ThinColumn` instead of `Column`. That means that
methods that previously returned `Column`, will now return `ThinColumn`
instead.
`ThinColumn` has a much more limited and low-level API, but you can
still achieve the same things in `ThinColumn` as you did in `Column`.
For example, instead of calling `Column::get_added_tick`, you'd call
`ThinColumn::get_added_ticks_slice` and index it to get the specific
added tick.
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Co-authored-by: James Liu <contact@jamessliu.com>
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