## Summary
While profiling agent sessions that make a lot of `edit_file`
operations, I noticed the LSP `textDocument/didChange` handler firing
excessively. Looking into this, I found out that the streaming edit
pipeline was applying each `CharOperation` from `StreamingDiff` as its
own `buffer.edit` transaction, and every transaction emits a
`BufferEvent::Edited` event. Each event can trigger several other
expensive events depending on whether the buffer is being rendered in an
editor or is registered with a language.
For example, there are `didChange` LSP events, the editor's on edit work
(matching brackets, bracket colorization, code actions, outline), and
more. A single `edit_file` could trigger hundreds of these at the higher
end in a single synchronous app update, which would block the foreground
thread for a bit and cause Zed to drop frames.
I fixed this by collecting all of a chunk's `CharOperation`s and
applying them in one `buffer.edit` call, so only a single
`BufferEvent::Edited` event gets emitted. This is safe because
operations are non overlapping by design of streaming diff (the edit
cursor only advances).
## Why this wasn't caught earlier
The cost only fully appears when a buffer is both registered with a
language server and rendered in an editor. Without that, most of the per
transaction observers never run, so the existing `edit_file_tool`
benchmark (which ran the tool against a bare buffer) didn't surface it.
I reworked the benchmark to open the edited buffer in an editor view,
register a fake language server with per edit diagnostics, and lay out a
frame, so it exercises the same cascade as the real editor. I also added
a larger fixture.
## Results
Measured with the `release-fast` profile on the reworked benchmark:
| Fixture | Initial file | Before | After | Improvement |
| --- | --- | --- | --- | --- |
| `tiny_function_rewrite` | 1.4 KB | 31.1 ms | 12.1 ms | −61% |
| `small_function_rewrite` | 3.0 KB | 42.4 ms | 19.3 ms | −55% |
| `medium_many_small_changes` | 4.6 KB | 309.2 ms | 151.5 ms | −51% |
| `medium_insertions` | 4.6 KB | 171.8 ms | 126.1 ms | −27% |
| `large_multi_edit` | 44 KB | 9,549 ms | 919 ms | −90% |
Self-Review Checklist:
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the UI/UX checklist
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable
Release Notes:
- Improved agent's edit file tool performance
Follow-up to #57772.
While comparing the `edit_file_tool_streaming/medium_insertions`
benchmark in Criterion and xctrace, I noticed the DP scoring loop was
spending most of its time on inlined `Matrix::get`/`Matrix::set` access
in `StreamingDiff::push_new`.
This changes the loop to split the previous and current score columns
once per DP column and then index those column slices directly. This
keeps the scoring algorithm the same, but hoists the column offset
calculation out of the inner loop and gives the compiler clearer
aliasing information between the immutable previous column and mutable
current column.
In xctrace, the hottest DP scoring line dropped from 10,407 samples
before to 9,409 samples after on
`edit_file_tool_streaming/medium_insertions`, and total
`StreamingDiff::push_new` leaf samples dropped from 10,800 to 9,822.
This is consistently faster in the end-to-end `edit_file_tool`
benchmark, and gives a small speed-up for the targeted workload without
changing the scoring algorithm.
### Results
Benchmarked with `release-fast` against a clean baseline using
Criterion’s `before_slice` baseline.
#### `streaming_diff_push_new`
| Fixture | Before median | After median | Change |
| --- | ---: | ---: | ---: |
| `tiny_function_rewrite` | `7.6317 ms` | `6.4951 ms` | 10.8% faster |
| `small_function_rewrite` | `45.961 ms` | `44.839 ms` | No significant
change |
| `medium_many_small_changes` | `99.694 ms` | `104.83 ms` | 5.2% slower
|
| `medium_insertions` | `95.909 ms` | `83.093 ms` | 13.5% faster |
#### `edit_file_tool_streaming`
| Fixture | Before median | After median | Change |
| --- | ---: | ---: | ---: |
| `tiny_function_rewrite` | `1.1110 ms` | `1.0617 ms` | 7.1% faster |
| `small_function_rewrite` | `2.4554 ms` | `2.2751 ms` | 7.1% faster |
| `medium_many_small_changes` | `87.272 ms` | `77.065 ms` | 11.5% faster
|
| `medium_insertions` | `92.469 ms` | `87.478 ms` | 5.9% faster |
The lower-level `streaming_diff_push_new` benchmark is mixed, but the
end-to-end `edit_file_tool_streaming` workload improves across all
fixtures.
Self-Review Checklist:
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable
Release Notes:
- N/A or Added/Fixed/Improved ...
