This should help with some of the memory problems reported in
https://github.com/zed-industries/zed/issues/8436, especially the ones
related to large files (see:
https://github.com/zed-industries/zed/issues/8436#issuecomment2037442695),
by **reducing the memory required to represent a buffer in Zed by
~50%.**
### How?
Zed's memory consumption is dominated by the in-memory representation of
buffer contents.
On the lowest level, the buffer is represented as a
[Rope](https://en.wikipedia.org/wiki/Rope_(data_structure)) and that's
where the most memory is used. The layers above — buffer, syntax map,
fold map, display map, ... — basically use "no memory" compared to the
Rope.
Zed's `Rope` data structure is itself implemented as [a `SumTree` of
`Chunks`](8205c52d2b/crates/rope/src/rope.rs (L35-L38)).
An important constant at play here is `CHUNK_BASE`:
`CHUNK_BASE` is the maximum length of a single text `Chunk` in the
`SumTree` underlying the `Rope`. In other words: It determines into how
many pieces a given buffer is split up.
By changing `CHUNK_BASE` we can adjust the level of granularity
withwhich we index a given piece of text. Theoretical maximum is the
length of the text, theoretical minimum is 1. Sweet spot is somewhere
inbetween, where memory use and performance of write & read access are
optimal.
We started with `16` as the `CHUNK_BASE`, but that wasn't the result of
extensive benchmarks, more the first reasonable number that came to
mind.
### What
This changes `CHUNK_BASE` from `16` to `64`. That reduces the memory
usage, trading it in for slight reduction in performance in certain
benchmarks.
### Benchmarks
I added a benchmark suite for `Rope` to determine whether we'd regress
in performance as `CHUNK_BASE` goes up. I went from `16` to `32` and
then to `64`. While `32` increased performance and reduced memory usage,
`64` had one slight drop in performance, increases in other benchmarks
and substantial memory savings.
| `CHUNK_BASE` from `16` to `32` | `CHUNK_BASE` from `16` to `64` |
|-------------------|--------------------|
|
|
|
### Real World Results
We tested this by loading a 138 MB `*.tex` file (parsed as plain text)
into Zed and measuring in `Instruments.app` the allocation.
#### standard allocator
Before, with `CHUNK_BASE: 16`, the memory usage was ~827MB after loading
the buffer.
| `CHUNK_BASE: 16` |
|---------------------|
|
|
After, with `CHUNK_BASE: 64`, the memory usage was ~396MB after loading
the buffer.
| `CHUNK_BASE: 64` |
|---------------------|
|
|
#### `mimalloc`
`MiMalloc` by default and that seems to be pretty aggressive when it
comes to growing memory. Whereas the std allocator would go up to
~800mb, MiMalloc would jump straight to 1024MB.
I also can't get `MiMalloc` to work properly with `Instruments.app` (it
always shows 15MB of memory usage) so I had to use these `Activity
Monitor` screenshots:
| `CHUNK_BASE: 16` |
|---------------------|
|
|
| `CHUNK_BASE: 64` |
|---------------------|
|
|
### Release Notes
Release Notes:
- Reduced memory usage for files by up to 50%.
---------
Co-authored-by: Antonio <antonio@zed.dev>
0533923f91